Are you sure you want to do this?
The preservation of the Quran was supported by Saduq, Tusi, Sharif Murtada, and [arguably] Mufid; and it is the dominant position (near consensus) in the hawzas and among our maraji`. Yes, we have had scholars that believed in tahreef, so have Sunnis. Tahreef is a complex discussion that Sunnis simply don't want to deal with, and so instead they catastrophize and hurl accusations at others. Al-Albani said that Abdullah b. Mas`ud (ra) did not believe that al-Falaq and an-Nas were a part of the Quran - that is tahreef. All Sunnis believe that ayat ar-rajm has been abograted only from the recitation (naskh at-tilawa) - this is tahreef. All Sunnis believe that the qira'at of the Quran (7 mutawatir, 3 ahad) do not holistically represent the 7 ahruf, which means that aspects of the 7 ahruf are either lost or jumbled into the current qira'at - this is tahreef. Simply research the basic differences between the 7 qira'at, and you'll find places where they differ in meaning (and not just in "accent" or "recitation" or whatever) - call it what you want.
The position of all twelver Shi`a, whether or not they believe in tahreef, is that the mus`haf today is the word of God. We must use it in our prayers, recite from it, and we even receive baraka by looking at it. It is the main criterion by which our ahadith are authenticated. The narrations on tahreef even tell us to stick to this mus`haf, and that the Mahdi will bring the correct qira'a once he comes. In other words, the practice of those who believe or disbelieve in tahreef is identical.
You quote these verses as though none of our scholars have read them in the last 1200 years. First of all, tawassul is not even a Shia issue, it is an Islamic issue. There are Sunnis who practice tawassul, and there are Shias who do not practice tawassul.
Tawassul is often compared to shirk, but to be honest, there will always be an intermediary between one and God. The names of Allah are created and separate from Him, and our words, actions, and rituals are all mediums between us and Him. As Imam al-Hadi once said, we share no medium with Allah, because that would be tashbeeh. We are creation, and we interact with creation - the Creator in His Essence is only known though His created signs. The Infinite God acts through the finite. Allah gives life, by allowing His created Spirit to breathe a soul into our bodies. Allah ends life, by allowing His created angel to remove our soul from our bodies. Allah delivers rizq to us through created means. So tawassul and ziyara, like salat, zakat, sawm, hajj, and other good deeds, are ultimately a path to God. They are all different means through which He is reached. A truly "direct" relationship is not plausible. As long as one believes that all created things have no power in and of themselves, then tawassul can only be an intercession to God, and not calling on a separate deity. Otherwise, the same argument can be made about all of our good deeds.
I'm curious to hear why you think istighatha in the Hereafter is so different from tawassul in the dunya.
I will simply quote my thread on Imamah in the Quranic worldview. InshaAllah you give it a fair and honest reading, along with its continuation thread at the end.
As for his point that the difference between Sunnism and Shiism is the finality of prophethood: this is disingenuous because many Sunnis (perhaps most) believed in maHfooth awliya' who performed karamat and kashf (infallibility, sainthood, miracles, and divine inspiration). It is wrong to assume that the Sealing of Prophethood = Cosmic Despair. The Seal even for Sunnis did not mean the end of divine vicegerency, as the Mahdi or even the Caliphs would fulfill that role.
The root difference between Sunnism and Shiism is the question of authority. For Sunnis, divine authority went to the Umma as a whole, which has the right to choose a divine vicegerent (khalifatullah), and the sahaba of which are all `adil and authoritative, and the consensus of the scholars is considered divine law, and the qiyas and ra'y of the scholars is acceptable in fiqh, etc. For Shi`is, the divine authority of the Prophet (s) passed to a man and not to the Umma as a whole. The latter is more Quranic, as it contains no concept of "people's rule" in divine matters; rather Allah's sunna is to appoint a vicegerent to lead the people, and demand that the people submit to that man. And Allah's sunna does not change (33:63).
May Allah keep our hearts steadfast on the wilaya of `Ali b. Abi Talib (as)