I think you'll find the answers to your questions vary drastically by species, as well as individual to individual snake.
1) my King and my hognose are fine with me touching their heads as ling as I come in from the right angle. My BP has gotten better with age but is still pretty head shy. My Borneo STP is super head shy, I don't really bother trying with him yet.
2) some are more tolerant of their mouths being opened than others, but I only do that if I need to.
3) my snakes seem fine with extending their bodies out, but I still make sure I've got a good handle on them. My Borneo is the exception, he doesn't wrap at all, so I make sure his body is fully supported at all times.