Nope, unfortunately neither of us could come to a solution for this. We tried everything but the only idea we could come up with is that something in the files outside of 'romfs' files is activating some kind of trigger that causes the game to crash whenever you edit any files that pertain to the adult content for any of their games.
I came to this conclusion as replacing some files works like a charm (hardly worth uploading though as the images don't make sense without the context), but the moment you try and edit any files that would enable the r18 content, crash on boot.
I believe it may in the exefs or something but I'm not well versed on editing those files, so the solution we both came to was to use the renpy homebrew with nsp forwarders to play the pc version on the switch via homebrew, which is more of a workaround but it does do a pretty good job.
Also, this mod for fts2 should be fully working now (minus that 1 hard locked cg), make sure you follow the instructions to the letter for this specific game, as the instructions and files needed are vary per game. Just be careful if you are trying to mod multiple games at once as you might mix up some of the files like I did at first
(I also reccomend not using the steam version as it is pixel censored and often more difficult to patch onto the switch than the other r18 patches)