Before trying to address your actual question I think it would be good to correct a fallacy or better a flaw in your argumentation:
You want salah to be explained in the Qur'an as it is a pillar of Islam! Or to stay closer to your words you ask why it isn't explained in the Qur'an while it is a pillar of Islam!
Now in fact we don't know about salah being a pillar of Islam by the Qur'an! We only know it from a few of many ahadith which taken seriously show that the pillars of Islam can be much more according to the different narrations or even there there's no consensus, however for some reason the narrations from ibn 'Omar and his father 'Omar ibn al-Khattab have been adopted by the sunni scholars as the main pillars of Islam. (See also Why is "doing righteous deeds" not a pillar of Islam?)
On the other hand if you expect the five pillars being mentioned in the Qur'an where are the two testimony's (a-Shahadatan)? There's no verse mentioning them both! And I even assume no surah in the Qur'an mentioned them both. They are only mentioned both in the sunnah! It seems that at least in the ahadith accepted by the Shi'a this doesn't appear as a pillar.
When it comes to Zakat about no ruling about it is mentioned in the Qur'an and it is certainly an essential part of Islam, as it gave the Muslims the right to kill people who reject paying it as they are considered as apostates!
From all of this we may already conclude that the Qur'an is the guideline but the Sunnah (the practice of the Prophet() and his interpretation of Allahs orders) gives the details and shows how to do an act. And for this we can find several evidences in the Qur'an, see for example:
... And We revealed to you the message that you may make clear to the people what was sent down to them and that they might give thought. (16:44)
... And whatever the Messenger has given you - take; and what he has forbidden you - refrain from. And fear Allah ; indeed, Allah is severe in penalty. (59:7)
So as the salah is an act that needs to be shown it clearly make sense to learn the how to pray from the Prophet() and it seems rather meaningless to discuss the details in a revelation as this would possibly create more confusion. And hadith and sunnah are the 2nd best the explanation of the Qur'an (see Is the Quran complete? If so, why do we need Hadith??) after that when the Qur'an explain itself (see examples for that in Is there any Quran-only tafsir (without Hadiths)?).
We can also conclude that the wives (explicitly) of the Prophet() and one can easily conclude that this applies to the sahaba too should transmit what they've learnt from the Prophet():
And remember what is recited in your houses of the verses of Allah and wisdom. Indeed, Allah is ever Subtle and Acquainted [with all things]. ([33:34])3
Some explanations
Now let me try to explain or give reasons why salat is not mentioned in detail in the Qur'an, but first you should know that the Qur'an has mentioned each position of the prayer this means we basically are informed about Wuquf (standing), ruku' (bowing), sujud (prostrating) and julus (sitting) and also that we might do dhikr at the later ones and that we need to recite Qur'an while praying. Therefore these acts are considered as fard (obligatory) by Muslim scholars.
As stated in my answer on Why do people pray salah differently? the scholars have different interpretations of ahadith, different sources and methodologies (even about the Qur'an see for example in Quran originally without vowel marks? the different interpretations of verse 5:6). Scholar A may consider hadith 1 as sahih while scholar B considers it as weak as it doesn't fulfil his conditions and so on. And as the sahaba have been spread in the Islamic countries their interpretation of the sunnah has played a big role on how later scholars would interpret and accept it for example the scholars of al-Kufa rely a lot on Abdullah ibn Mas'ud, while those of al-Hijaz rely on 'Abdullah ibn 'Omar, in a-Shaam they would rely on sahib such as 'Abdullah ibn 'Amr ibn al-'Aas and so on. In case of consensus no Scholar would refuse the interpretation of a sahabi, but in case of difference they would consider it as an other interpretation or an other approved option on how one would be able to do something.
So if the Kufi scholars say we don't need to move the finger during tashahud unless we recite the name of Allah, this is acceptable and if they put their hands under the naval it is acceptable, if other scholars who follow the sahaba and tabi'yn who considered that putting the right hand on the left is makrooh for imitating ahl-al-Kitab is also acceptable, saying one single tasleem is also acceptable as there are many sahaba who used to end the prayer with one single tasleem. However it is always best to have a backup in the sunnah of the Prophet() as nobody knows better how to pray.
Is it important to stick on the tashhaud of 'Abduallah ibn Masu'd? No it is one out of many formulations which have been approved by our Prophet (), so there's in fact no one way how to pray, as you may be able to vary your prayer for example by saying du'a while prostrating instead of the known tasbeeh (subhana raby al-'Ala) etc. you may vary this tasbeeh.
Note that scholars say that the difference of opinion among scholars is a benefit and mercy for the Ummah, as it opens rather closed doors. Also if you want to concentrate in your prayer it actually is recommended to vary your recitation of Qur'an, tasbeeh, du'a etc. as else your prayer would be like a repetition of sequences and that would hardly help you to concentrate and reach pure khushu' خشوع which is the highest level of prayer:
Certainly will the believers have succeeded: (23:1)
They who are during their prayer humbly submissive (23:2)
So having multiple options in how to pray seems to me rather a help to concentrate and reach a state of khushu' in the prayer rather than creating uncertainty, as uncertainty always is there in any act of worship you do you are uncertain whether Allah will accept it or not. And the m ore you doubt it will be accepted the more you might try to do it better!
It also is meaningless to discuss how to hold hands or not or how many tasleems must be done or how often one needs to raise his hands, as long as one can concentrate as the main goal of the prayer is actually not to do some exercise rather than to concentrate on your communication with your creator and our prayer should in first place prohibit us from immorality and wrongdoing and we should take it as a five time review and communication with our Lord. And this can only be achieved by remembrance of Allah not by a standing or sitting position or a position of our hands or feet etc.
