Regarding the tashahud issue:
There are very few religious issues that Muslim scholars pretty much don't differ about at all. As far as I have read, the obligatory parts of prayers (like the tashahud) are among those issues. The reason being that such practices were being performed by thousands of Muslims in Makkah, Medina and other areas, therefore there was little doubt about how these religious practices were supposed to be performed.
If the evidence or proof regarding the obligatory parts of salat is not enough for a Muslim, then if I'm not mistaken, the evidence or proof for the reliability of the Qur'an (from a purely historical point-of-view) should also not be enough for that Muslim.