Some things I agree with, others I don't. I don't want to cut your post up, then go back and forth because these usually turn into books.
But I'll make a few points.
Shia's are somewhat of a diverse group. Obviously, those who beat themselves or call upon Ali (ra) are upon clear deviance. But not all of them have these beliefs like the infallible 12 imams or w/e.
This goes for all the sects, Salafi, Deobandi, etc. Unless there are clear signs of kufr like Ahmadis who believe in a prophet after Muhammad (saw). How can we say they are not Muslim?
We need to be careful with takfir. If you call a Muslim a kafir, and he is not, then it comes back to you. The signs must be absolutely clear, lest you risk leaving Islam yourself.
Sorry, I don't know who the guy in your twitter link is.