backwards compatibility is definitely gonna happen with Switch2. it happened with the DS and 3DS (and also a few other Nintendo consoles whose names i can't remember), i see no reason why it wouldn't happen with Switch2.
Squidboards
TOD13
PacManFever
While I think it's most likely going to happen, I wouldn't say it's definitely going to happen. All of the exclusively handheld consoles had it (except the GBA SP and DSi) because it was cheap enough to put the older chips inside the new consoles. This was not the case for the 3DS. At launch, it cost $250, which Nintendo then dropped to $170. They had to sell the 3DS at a loss due to poor sales.