My reason for asking is I believe the promotion to be instant: As the pawn's toe crosses the line into the square, it transforms into the promotion piece's toe... Meaning that even though the route was blocked for a pawn, the promotion to a queen (for example) would allow the move to proceed.
No. A pawn gains no special movement rules when on the penultimate rank. Often in end-games the opposing King will be in front of the pawn to prevent it from being promoted.
This is not football, where a "touchdown" is scored by crossing the "plane" of a goal line.
In order to become a queen, a pawn has to occupy a square on the eighth line. If that square is already occupied, a pawn can't "share" the square using a forward move.
The only way a pawn can queen against an enemy piece on a queening square is by capturing it with a DIAGONAL move.