In college football can a defensive player force a receiver out of bounds while the receiver is in the air or would this be ruled a completion because the receiver would have come down in bounds without the defensive player
pushing them?

Only a reception if you rule he was carried out. If he was just knocked out, then no catch

True, but he has to be held so he is prevented from returning to the ground. But this doesn't really have anything to do with be "forced out." Its that, by rule, the ball becomes dead when the receiver is in such a position that he would be carried, and if he's in possession of the ball, its a catch.

There's still no real "force-out" rule in NCAA.

Rich, you are referring to the IA game, and it was the correct call. He was not carried out, but hit out. That is a rule specific to NCAA.