The photos of 256×240 with reverse LPF look like you didn’t tweak the sample phase perfectly. Set reverse LPF to 4 and tweak sample phase some more.
Your RGB cable indeed looks like a cheap composite video sync cable. I have a bunch of those purple ones from back in the day because they were so cheap. Looks like yours has been modded for PAL.
Its best to avoid cables with sync strippers. Its really only needed if you have some device that requires clean sync but luma sync will work without issues on the OSSC. The standard luma sync cable is fine but if you want to avoid audio interference you can buy the shielded packapunch cable.
For the different SNES models you only need to consider that PAL uses a different cable than a NTSC SNES. I also would not recommend using csync for your NTSC SNES as it can give a lot of problems. One of those is that a PAL SNES outputs 12 volt on the same pin so if you accidentally switch the cable you might damage the OSSC.