I don't think he's obligated to support any issues caused by custom roms. But, he is doing really well, fixing lots of issues for many devices and games so if he hasn't managed to fix your issue then either: a - He's already aware and working on it; b - Doesn't have that phone and rom to be able to ...
It should be in the same path. But, each disc has a different game ID so Disc 2 will also have a different save name. If you're using the easy mode, you should be able to see what the Game ID is for each disc
Extract the .7z file and you will get the files that you need. Not sure what you mean when you said you use the gba emulator fine, that is a completely different product, different creator, different file types etc. Also, can you edit your post to remove the name of the site used for acquiring the r...