By definition, as I was taught again, if a player leaves the field via a goal line he is deemed to be on the field at the goal line. In the case of an offensive player whose momentum carries him over the goal line and two defenders cross over as well, the player could probably return and be onsides since the two other defenders are deemed to be on the field on the goal line.

If you're within a step of the goal line, the referee isn't going to penalize you for returning to the pitch. If the attacker has run to the ad board because he chose to and became a spectator, he should be booked for leaving the field of play without permission from the referee and cannot return until beckoned. But being run off the pitch because you're chasing a loose ball, for example, isn't an issue. That's just normal play. Walking off is different.

That happens quite often, and it's just considered to be the normal course of play.

Don't overthink the impact to offside in this scenario. If you're off the pitch beyond the goal line, you're considered to be ON the goal line w/r/t offside positioning. However, you obviously cannot impact the play if you're off (unless you're just off, then you can, in theory). If you come back on, you can clearly impact the play.