Frankly those nuances happen because the league is trying to make compromise rules to protect player safety. What other good reason is there for the forward progress rule anyway.
The forward progress rule is as old as God, established long, long before the league cared much about player safety.
The rationale for that rule is simple. Without it the defense could pick up or hold up a ball carrier and move him back instead of tackling him, theoretically as far as the end zone for a safety.
The scenario you describe only applies to penetrating the goal line and is not a forward progress ruling per se. The rule states that if the the ball crosses over any point of the goal line under ball carrier control before he is down, the play is immediately over and a TD is scored.
This blanket rule avoids problematic interpretations. Without it, how do you handle a ball carrier reaching inside the pylon, crossing the ball over the goal line, but who is "down" out of bounds? Where do you put the ball if it is not a TD?
It's a simple, all purpose rule that handles a lot of situations, and it too is as old as God.
Oh, geez, that was way off topic, wasn't it? I guess I'm heading for a self- suspension.