I'm curious, why is mono preferred when playing live? What problems does stereo introduce?

There's a few problems. Venues are often used to dealing in mono because the majority of the acts will operate that way (guitars etc) rather than in stereo.

It shouldn't really be an issue tbh, and any decent sound engineer/venue should be able to accommodate stereo signals, but it isn't always the case - especially in smaller places. As a result, it can wind up safer to just make sure you have a mono output to the desk so you don't lose out on the notes that are panned to one side or the other.

There's also the train of thought that the stereo panning doesn't really work well in the live environment anyway, since people will be stood all over the place - so won't get the benefit of the effect. It can end up with them getting an unbalanced experience of the track if they are stood nearer one speaker stack or the other.

Do I need to make a new .sav with all "O" commands removed as well as all instruments LR?

Do I put all channels on the song screen to LLLL?

Please and thank you.