Let's start by examining and discussing some of your examples:
1) The Quran does not say WHERE the first ayah was revealed to Muhammad. So the location of the first ayah is irrelevant to the Quran. Yet, Muslims believe it was revealed in the Hira cave in Makkah. Now suppose a muslim says "The first ayah was not revealed in Hira cave. Muhammad was sailing in the Indian Ocean and then the first ayah was revealed there". Will Allah punish this person? I don't think so. Because as I said, the location of the first ayah is irrelevant to the Quran.
It is true that the Quran doesn't clearly say that the first revelation was in the cave of Hira'. But Muslims wouldn't believe it if it was not a well-known historical fact. Which you might find in all biographies of the Prophet Muhammad (). Note that some of these reports came from non-Muslims also, so it is definitely a historical fact. If you rejected it then you could also reject any technical inauguration or whatever history tells you.
The rule here is if anybody says something which is not true and they know it is not (true) they will be punished by Allah, if they didn't (know and said so) Allah’s justice would result in forgiving them or punishing them for claims they've made without knowledge (17:36).
2) The Quran does not name the wives of Muhammad. But if a muslim said "Khadija was not the wife of Muhammad" will this muslim be punished by Allah? I don't think so. Because the identity of Muhammad's wives are irrelevant to the Quran.
Again we know the names of the wives by many sources: historical sources (I think I've made my point clear above) and hadith also tells us about them and their marriage. If you pretend that Khadija was not his wife and you know it from these sources you'll be either punished for telling lies (or maybe even for qadf) ...
Note that the Quran at least tell us about the wife of Zaid whom our Prophet married later and make clear who he is and by this who she is, Quran also teaches us that our Prophet has been telling a secret to some of his wives...
The Quran itself also uses the plural (not the dual plural) form for wives so we are sure that Muhammad () has been married to at least 3 wives and concubines (note that they are quotes separately, so one cannot say 3 = wives + concubines)!
3) According to the hadith, Muhammad said "For him who says: `Subhan-Allahi wa bi hamdihi, a palm-tree will be planted in Jannah." So what happens if a muslim says "I don't believe this hadith. If you say this supplication, Allah will reward you but he will not plant a palm-tree in Jannah"? Will Allah punish this muslim? I don't think so. Because whether a palm tree will be planted in Jannah or not is irrelevant to the Quran.
I don't know of such a hadith and if it is said it would be a good deed to follow it not because of the tree in Jannah or the statement of the hadith but because of the remembrance of Allah by saying these words (Quran indeed invites us to praise and thank Allah). So this hadith goes along with the teachings of the Quran, you can reject it if you have your good reasons and you might not be punished, but as said this is one part of the teachings of the Quran if you rejected it you could be indirectly rejecting the Quran too.
4) The Quran does not say how to pray Salah. We learned the techniques of Salah from the hadith. But if a muslim says "All the ahadith about Salah are wrong. You should only pray salah in your mind", will Allah punish this Muslim? DEFINITELY. Because praying Salah is definitely relevant to the Quran. And Muhammad knows best how to pray Salah.
If he said so then he doesn't know or understand or haven't read the Quran well. Because if he had read it he would pass by the verses of surat an-Nisa' (101-103) which clearly say that salat is not in our minds only. So at least prostration doesn't come from hadith only, but I'm pretty sure that not only my interpretation of these verses clearly says that one must pray standing, bowing and sitting also (note that even if this might not be explicitly defined as part of the prayer in this verses the amount of repetitions once "prayer" is quoted in a verse should be strong hint!).
Do we need to follow all the (authentic) hadith and what if we reject all the hadith
If you want to know why we have hadith beside the Quran or why the Quran is not enough you might read this Is the Quran complete? If so, why do we need Hadith?
If you want to know more about rejecting the hadith you might read this What is the ruling regarding Hadith Rejecters? and this What kinds of sin does hadith rejection lead to (if any)?.
One can discuss the use of many ahadith one can even reject some ahadith if they clearly contradict the Quran as a hadith should never contradict the Quran. But if you rejected all the hadith you would (indirectly) reject the existence of our Prophet and his teaching and this is a historical fact that nobody in the world rejects no matter if he believes in his teachings or not! So by this one would be even worse than a kafir!
If you said many scholars tried to interpret ahadith in the sahih books or other books in such a way that they may go ahead (or make them fit) with the Quran even if they clearly contradict it I would agree there are some examples that really give a bad impression of Islam in the Sunnah and also in some interpretations of the Quran these need to be re-discussed and corrected!
Scholars should have the courage to point at them and discuss them I'd be glad to live to see the day where a scholar would publish sahih sahih al-Bukhari and sahih sahih Muslim etc.!
Many scholars of hadith sciences have spent too much time on discussing the authenticity of ahadith based on the narrator chains and have forgotten the content or just tried to find a "fit" to let the content go ahead with the Quran and other basical statements of the sunnah. Imam Malik said something like: "This science is a religion so examine from whom you take it from" I would correct this statement and say: "Examine from whom you take it and what you take and check whether it goes ahead with the teachings of the Quran" as this is the way he has threaten the sunnah: in al-Muwatta' Imam Malik even quoted a lot of ahadith he didn't rely on in his madhab, as he found them contradictory to stronger sources like the Quran or the practice in Medina!
