The scripture of the mohsaf al-Imam
Let's start by commenting a few points:
The scripture or style of the Quran written at the time of 'Othman is called the Othmany style الرسم العثماني or الخط العثماني, which is basically one form of what was later called kufi scripture. Later because at the time al-Kufa didn't exist as it was founded by the orders of the Caliph 'Omar ibn al-Khattab as a fortress near the place the writing style moved from to al-Hijaz centuries before Islam, after coming there from al-Yeman: the Mesopotamian city called Al-Hirah الحيرة.
So correctly speaking kufi scripture is a wrong definition, but the fact that al-Kufa was later known for copying books and moshaf's until the 4th hijri century established the name of this writing style!
Abu al-Aswad ad-Du'ali added points or marks in the moshaf copies to mark some vowels and intonation one could even say that he was guided by Ali ibn abi Talib (). This was kept until further styles of writing and calligraphy came and diacritic was needed to help people read in a moshaf (I'd recommend you to take a look at the pictures on the Arabic wikipedia site to get a first impression, here a link with explanation).
In fact abu al-Aswad ad-Du'ali converted to Islam during the lifetime of the Prophet(), but never met him, so he is strictly speaking not a sahabi and al-Hajaj ibn Youssuf a-Thaqafi -the despot of the times of the Umayyad- relayed on his students to establish a new system based on red dots that show the kind of intonation.
Tashkil (diacritic) in it's first form -not exactly as we know it now- has only came to life in a century later by al-Farahadi based on further developments by scholars from other places like al-Basra and al-Medina.
Note that this addition could have been considered a bid'ah by scholars of the time as it was a change in the original text. For example: In my family we have a handwritten moshaf who doesn't include any verse numbers as these are additions to the original text, but even that moshaf would have been considered as including bid'ah elements.
But to be honest these diacritics and many other improvements are and were a big help to spread the word of Allah, so the bid'ah consideration here can be discussed on the basis of the knowledge of the early times compared to our Arabic knowledge nowadays, many Arabic people are not fluent in fasih -modern- Arabic! And without these tools many non-Arabic people of the time would have big differences to learn the Quran. And the canonization of the moshaf al-Imam at the time of 'Othman was made with the intention to keep one canonical scripture which represents the most authentic way to recite the book and help non-Muslims and non-Arabs to learn it more easily and not to mistake while reading or reciting it.
Quran is the recited and orally-transmitted word not the text nor the book
But the most important thing to know is that the Quran is only the oral transmitted text not the book, so what ever is inside of this book might be a change of the original scripture, which would be considered as a bid'ah by earlier scholars. None can be called hafidh/hafiz unless he has learnt and memorized the Quran from a former hafidh, else scholars would speak from a moshafi a person whom learnt and memorizes from a book or papers. Until today many people seek for scholars with a connected narrator chain of a qira'a until our Prophet () the shortest chains would have 27-28 intermediates! The same applies for hadith collections! As it seems among the Imams of the 4 sunni schoools of jurisprudence Imam Ahmad was the one with more knowledge of this kind as he at least knew the kufi-qira'at and the one of Medina. While Imam abu Hanifa was only familiar with kufi qira'a -the 'Othmans therefore helped the riwaya of Hafs to get a leading position in the Ummah-, a-Shafi'i with the Makki and Malik with the Madani qira'a.
Shi'a consider the reading of hafs as the most authentic, maybe because they heard it is on the authority of 'Ali ibn Abi Talib, but the fact that hafs includes lots of terms and elements which are related to ibn Mas'uds hudahli dialect instead the hijazi dialect of 'Ali ibn Abi Talib refutes the idea of it being purely a narration of 'Ali, beside the fact that shi'a primarily relay on sunni sources when it comes to qira'at (there first source is ibn Mujahid whom "invented" the 7 qira'at by choosing some and leaving others and maybe was the cause for others to vanish). And I would wonder if the Imam's who grow up in Medina would have ever read or heard of the reading of hafs 'an 'Asim as this reading was not even well known in al-Kufa itself!
If we leave out all vowel marks from the current canonical text, are there passages that become ambiguous?
Here different views must be considered:
- For somebody who isn't good enough at Arabic even the vowel marks are irrelevant, as they would make mistakes in the recitation I've really heard some awful misinterpretations even by Imams of non-Arabic people!
- For a person who can read Arabic, but don't understand it well, it would be the same.
- For a person who is familiar or even fluent with the language a few words might become ambiguous.
- For people who have memorized the Quran the correct way that wouldn't even bother them.
The second part of your Question might refer to ahadith such as the one narrated by 'Attiyah, Abu Sa'id al-Khudry, by ibn Jubair from ibn 'Abbas and by ibn Shihab from ibn 'Abbas in Jami' at-Tirmdihi... in one of these he (at-Tirmidhi) quotes in a comment a recitation of Nasr ibn 'Ali نَصْرُ بْنُ عَلِيّ who recites it in active and passive form (indirect relation to vowels):
(transliterated) ghalabat wa ghulibat
(meaning) has defeated and has been defeated (afterwards)
This recitation is explanatory and not or no more considered as valid after the consensus on the Moshaf of 'Othman. But it is helpful to understand things and meanings. The consensus of the valid qira'at is on the passive form (غُلِبَتْ) the Rum have been defeated!
To answer your former question based on this verse: One can say a good Arabic speaker (even of our time) would conclude the correct intonation form from the context as the 3rd verse of this surah uses both the past and future form the active form wouldn't make sense. But of course if he/she doesn't have yet read the coming verses there would be the two possibilities.
Does early Islamic scholarship discuss these issues?
Imam Malik used to answer questions about the Quran by ask Nafi' (as they were contemporary scholars) on each science one needs to ask the specialists (this for example was his answer on if the basmallah is part of the Quran or the fatiha only or of each surah). However Malik as quoted in some of my answers on the following links beside many (if not all) scholars gave fatwa of not changing or adding something to the original rasm of the moshaf as they considered it as canonical!
If you'd like I can add some well discussed verses which are mostly discussed because they are related to orders or prohibitions and have different readings (qira'a) not necessary because of a vowel issue! But give me some time.
Here some useful and related links:
How many writing styles of Mushaf Quran that exist?
What are the readings (qira'at) of Quran?
How to explain the recitation لَئِنْ انجــينا (verse 6.63) by Al-Azami in his book "The History of the Quranic text"?
Different versions of Arabic Qur'an
Why is the Hafs reading of the Qur'an so prevalent?
How many spellings do exist for Quran's text? How many rasm-al-mushaf do we have?
Examples of verses of the Qur'an with two different readings?
What is the literal meaning of the word "Qur'an"?
OFF-Topic: Let's get granular with verse 5:6
Based on tafsir al-Qurtobi I'll shortly comment on some issues in the following part of the verse:
O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles.
before, while I'll get granular when it comes to differences in the readings:
Note that the translation from above already shows a single interpretation of the verse, which needs to be distinguished according each correct reading!
- "when you rise to [perform] prayer" has been taken literally by some sahaba namely the 4 caliphs, 'Ali has even been quoted quoting this verse as an evidence: they performed wudu' (ablution) for each prayer, while the sunnah shows that the Prophet () performed consecutive prayers with one single ablution! So whom ever follows this view might try to follow the best deed, but whom doesn't perform wudu' for each (mandatory) prayer is still following the sunnah. As this is also considered an abrogated act (performing ablution for each mandatory prayer).
- "wash your faces" is also somewhat discussed, for example is the beard part of the face, are nose and ears and mouth part of the face, if yes are they part of the fard (obligatory) washing or not and to what extent is discussed!
- "and your forearms to the elbows" it is discussed whether forearms are part of the mandatory washing or not. Abu Hurairah has even been reported to be completing his washing until his armpits, saying our Messenger () has recommended that to him, al-Qadi 'Iyad has disapproved and rejected this statement, the Maliki's consider intentionally washing more than the necessary mandatory parts as makrooh.
- "and wipe over your heads" here scholars have differences because of the Arabic letter "baa' باء" in برءوسكم (over your heads) which might represent a part of the whole, so some consider wiping over a part of the head as valid (enough), but they differ on how much this part exactly is, al-Qurtobi finally says the best is to consider wiping over the whole head!
- "and wash your feet to the ankles" as said this translation shows one single interpretation of "وأرجلكم" (and your feet): The main issue here is the letter "waw واو" (which is a synonym for "and") and it's implication on how we would pronounce or read the word feet (arjul), depending on this we would say feet follows the ruling of washing as it follows the grammatical rules of its predecessors face and forearms or it follows the the ruling of wiping as it follows the grammatical rules of its predecessor head!
So if we read it "وأرجلَكم" (transliterated: wa arjulakum) the letter "waw" (=and) refers to washing as the word "arjul" has the fatha diacritic over the last letter "laam لام" so it grammatically follows the faces and forearms. This is the reading of 'Asim, Nafi*, ibn 'Amir and al-Kissaa-i.
If we read it "وأرجلُكم" (transliterated: wa arjulukum), with a "dammah" over the "laam" this is said to be a reading of Nafi' and al-Hassan and al-'Amash and seems a singularity as it is not or no more a valid reading.
But if we read it "وأرجلِكم" (transliterated: wa arjulikum), with a "kasrah" over the "laam" which would grammatically follow the word heads and therefore the ruling for wiping over it. This is the reading of ibn Kathir, Abu 'Amr and Hamzah. Note that wiping over leather socks (khuf) is allowed according to some circumstances, so basically this reading could include this interpretation. Among sunni scholars only at-Tabari approved this interpretation (allowing both washing or wiping as there are two different readings), while the majority considers washing the feet as the mandatory act! There are ahadith showing that the sunnah is washing them however Anas () rejected a speech of al-Hajaj ibn Youssuf saying that there are two washings and two wipings in the Quran at the beginning an then explaining that the parts of the feet need to be washed. Saying: "Allah has told the truth, and al-Hajaj has lied صدق الله وكذب الحجاج" and in his practice he wiped while dropping water on his feet (which is considered as washing by most scholars). Ibn 'Abbas () confirmed that the Quran came with the wiping and the sunnah with washing.