So should I definitely forget about using RGC’s solution directly or with an UMSA, for the interface-to-Trinitron TV set connection?
I’ve done similar in the past and it’s been fine, but I didn’t realise it was out of spec (assuming I’m understanding everything correctly here of course), which could cause wear and tear to your CRT, so I guess the choice is yours.
Putting the Extron interface output through an UMSA would be safe since that would (or at least should!) correct the signal down to SCART spec levels, but people have been criticising ArcadeForge’s customer support lately which is a shame as they make some useful hardware.
As for the S-video thing, a PS1/PS2 luma-sync RGB SCART cable (+ adapter) won’t do for using the RGB signal on the interface?
No, the interface cannot accept anything other than clean sync, simply is not equipped to handle it.