Rollback is also CPU intensive. The simpler things are, the easier it is. But the more stuff you have going, the more things need to be taken into account. It's not just physics: you have to consider sounds, the various command inputs if different players for example: all info that is being registered needs to be accounted for rollback to work. Depending on the degree to which rollback is used, more frames can be kept and adjusted.
The Nikelodeon fighter supposedly only has rollback for 1v1 on the Switch. 2v2 doesn't support it:
Additionally, 'emulation' is most likely being used to account for the game having to emulate what happened before, and then to emulate what the outcome would be (hence rollback)
If the Switch is unable to support the same level of rollback being seen in the other versions, then that itself will be a problem for cross-play. So, they either 'ruin' the online experience for everyone because Switch users can do cross-play, or they close-off the Switch version as it's own thing. Neither is ideal situation. If anything, they'll just wait for a stronger Switch to release.
Sorry no, you don't need to calculate sounds, frames, graphics or any of the like, as all outputs are already done and cannot be changed. What you roll back is the position of the fighter and the action that did happen. If you calculate all the other stuff, you do something wrong. If the game is emulated and you add rollback outside the emulation (because the original game was only local and you add networking outside of emulation by simulation the remote player as player 2), then you have to rollback the whole simulation, no way around it unless you change the original code, which wouldn't be emulation anymore.