As for the proposed rule change, I think there are some weird situations that could arise out of it too. Like what if a defensive player intercepts the ball, then fumbles the ball out of bounds in the end zone during a hit while trying to run it back? Or if they recover a fumble in the end zone and then fumble it on their own after claiming possession? Or on a kickoff that gets pushed out of bounds? There are several other rules that would require some adjustments if you wanted to implement this one.
I don't think it changes anything at all. We're only talking about changing what happens when the ballcarrier fumbles through the
opposing team's EZ, not when he fumbles through his own.
For reference, in the cases you mention here's what the
existing rules say:
1A) Defense intercepts the ball in the EZ, fumbles it OOB in the EZ (or is recovered in EZ by intercepting team) while running it back. This is a touchback and intercepting team's ball. It's the same as if the defense intercepted the ball in the EZ and took a knee. The offense provided the impetus putting the ball in the EZ via the intercepted pass. Because the ball never left the EZ, impetus cannot be "reassigned" to the returner, so it's touchback and not a safety.
1B) Defense intercepts the ball in the EZ, comes out of the EZ, then fumbles it back into and through the EZ (or is recovered in EZ by intercepting team). This is a safety. The ball was out of the EZ and the returner provided the impetus that put it back into his own EZ. So safety.
2A & 2B) Exactly the same as for the interception cases. If the ball never left the EZ, touchback and recovering team's ball or a safety if the ball had first left the EZ.
3A) Returner is in EZ. Kickoff or punt hits him there and goes OOB in EZ (or is recovered by kicking team). Touchback and receiving team's ball. Same as if they caught it in EZ and took a knee. Again, kicking team provided the impetus to put the ball in the EZ and because the ball never left the EZ, impetus is never "reassigned" no matter what happens to the ball.
3B) Returner is in the field of play. Kickoff or punt hits him there but ball continues into EZ and goes OOB (or is recovered by kicking team). Still a touchback and receiving team's ball because muffing a loose ball generally does not "reassign" impetus. There needs to be possession, clear bat of the ball, or muffing of a ball that has completely or almost completely stopped moving for impetus to be "reassigned".
3C) Returner is in the field of play. Gets possession of the kick, then fumbles and ball continues into EZ and goes OOB (or is recovered by kicking team). Safety. Possession and subsequent fumble "reassigns" impetus.
I don't see why any of that -- which is all about what's happening in a team's
own EZ -- would need to change if the rule is changed about what happens when a team fumbles through the
other team's EZ.
(BTW, impetus only matters when the ball goes OOB in the EZ or is recovered by a member of the team who that EZ belongs to. If the ball is recovered in the EZ by the team that EZ does not belong to then it's a TD regardless of whose impetus put the ball in the EZ. In other words, impetus only matters if the play isn't a TD. If it's not, then you go to impetus to determine if it's a TB or a safety.)