NFL has very strict numbering requirements. If a player wearing an ineligible number (50-79) lines up in any eligible position (ends of line or backfield) without reporting to the referee, it is a foul. Similarily, if a player wearing an eligible number lines up on the line between the ends, it is a foul. Most often you see the TE accidentally covered by the WR, although there was just now a play where a 50-79 numbered player had reported and was covered up by a WR.
1. No idea for the reason.
2. You can have more than seven player on the LOS if all interior linemen are wearing 50-79 or have reported.
3. Yes.
4. No. In NCAA the requirement is to have at least five players wearing 50-79 anywhere on the line, no requirement for the backfield or ends of line, and players wearing 50-79 cannot report to the referee to be eligible receivers.