1) retrofw is the best firmware for RS-97 has the most optimized and updated emulators? I know the native system, the dingux and the retrofw
Yes, use RetroFW 2.1 in particular. You can use the OPKs and my older IPKs releases as well. (Personally i'm slowly updating them but it takes time)
RetroFW 2 also supports the IPU, which saves a ton of CPU time that would have be spent for upscaling or downscaling.
(So you will gain 3-4 FPS in some emulators)
It also properly supports TV out, something that was removed from the first RetroFW release as it was temporarily removed.
2) is it possible to find out what is the board version of my RS-97 Plus without opening the console and removing the screws?
If the screen doesn't have smooth movement or just tears in general, you have the bad RS-97 Plus.
Otherwise, you most likely have the good one. I mean, the horrible screen is pretty much noticeable.
I believe there are other differences as well but i forgot.
3) is it possible to install the retrofw on the internal card without having to open the console and remove the screws? install via USB cable
It is possible on the older RS-97s revision but for some reasons on the consoles with the JZ4760B soc, they disabled the feature to flash it from USB. (This could have been fixed on the IPS models but not sure)
RetroFW added some sort of USB debugging but last time i tried it wasn't reliable.
Looks like you will have to take out that sd card, that's what most users do.