Kerry Collins
Yes. As long as you do not touch the receiver of the pass or block his/her's vision of the ball. If you do touch or 'face guard' the opponent then you will be called for pass interference.
The entire foot must be in bounds.
No. The ball is dead as of the illegal touch.
Flag football or touch football
In Australia, the game of "Touch" is played with a football. It is sometimes incorrectly called "Touch football", but it has no allegiance to the game of football.
"touch rugby"
Yes, you can reach out and touch another half of something.
In an on-sides kick the football does not have to touch the ground. The football must travel at least 10 yards before the kicking team can legally touch the football.
NRL is tackle football, there are kicks in play (start of field) and there are converisons. Touch football requires you to touch a player, no kicks during plays and no converisons.
Tap: dummy half caught, off site, change over
Offensive lineman are not allowed past the line of scrimmage prior to a pass. He could catch a screen pass behind the line though. Or they'd have to declare him eligible before the play. When I played flag football we used to do center sneaks. The ball has to touch the QB hands to be considered a hand off though. I don't think that would work well in tackle football, but in flag football where they only had a couple people rushing you could easily get 5-10 yards before anyone noticed.