Well we also need to consider that every last-minute change the NFL makes needs to also be approved by the NFLPA, and while I obviously can't speak for the PA, I believe they generally disapprove of players playing additional games, because more games = more danger to the players.

This. Everything is set in stone like a year in advance, the Super Bowl is not moving.

Genuine question: How is the viewership of these Nickelodeon NFL games? I know they've only had a handful of Nickelodeon broadcasts in the ~3 years it's been a thing, but are there any viewership stats indicating that this is working to get young kids interested in the NFL?