Wow, a lot of questions. I'll try...
How many songs? Right now about 20-30, plus a handful more that I only practice and play during the Christmas season.

I know that the OTS are contained within the styles, which means at least one custom style per song, stored on the USB stick.

If I intended to use "next performance"-type functionality exclusively (instead of OTS) to change voicing, style variations, etc., most likely there would be more than two performances per song.

To obtain a momentary change of the performance, I'm using a MIDI Solutions Dual Footswitch Controller module. A "black box" between the Boss FS-6 and MIDI in. It's programmed to call a specific performance number on pedal press, and returns to the base performance on release. One limitation is that a limited selection of MIDI messages can be sent on release; one of them is Program Change. I set this up with a base performance and two alternates. One of the alternates has Bass Inversion enabled, the other has 'Standard' chord recognition (instead of Intelligent) to permit the playing of "open" chords. Releasing either pedal on the dual footswitch immediately returns to Intelligent mode.

The FP-50/FP-80 has Bass Inversion programmed as a momentary function using the pedals. This is very useful in real time play (much faster and more reliable than pressing the pedal once to turn Bass Inv on and again to turn it off.) Also, most Roland arrangers going back to the G-series have had built-in footswitch functions to "toggle" chord recognition between Piano Style and Standard, or Piano Style and Intelligent. What I'm doing is toggling between Intelligent and Standard. I probably could have taken a different approach if Roland recognized [1+5] as an open Fifth in Intelligent mode. But it doesn't. And the songs that call for power chords just don't sound right without them!

The easiest way to gain the above functionality and not have to embed these changes in successive performances several times during a song, is to embed Bass Inv and Arranger Type in top-level performances so the foot switches ALWAYS do the same thing. (This also facilitates learning and practicing new songs that include these types of chords.) Once the song becomes part of my "gig list," the rest of the song-specific changes will be stored as a customized user style and complementary OTS.

So... is it possible on a BK to call a specific style number from the USB stick with a MIDI message? Even if the style has to be stored in the root folder? If so, there might be iPad apps or an outboard sequencer that would allow direct selection and reduce the "scrolling and pressing?" Just thinking aloud here.