Longstick B44 leaves Zone 1 and heads toward the midfield line. While still in Zone 2, B44 is stick checked by a Team A middie which causes the ball to "pop" out of B44's crosse and back into Zone 1.

The ball never touches the ground in Zone 2.

The ball is caught/fielded by a Team B player in Zone 1. Is this illegal reentry?

What if it hits the ground in Zone 2, rolls into Zone 1 and then fielded by Team B?

Biz...in your first scenario the ball is checked out of the D man's stick after he has crossed the def. clear line (box)
and it lands directly back into Zone 1. This is re-entry. If it becomes loose in Zone 2 (apart from being deliberately turned loose)..then it's back to ground zero and it's not re-entry.