You are almost there
Run Triangle Away and get to Official status then just install the stock recovery through Odin. That should be you in good shape.
Apparently, the problem with MG4 is that it changes the efs partition structure. That is the reason you can't install older roms. As far as I know, the only way to go backwards is if you have an efs backup from before MG4 installation. You can then restore the older efs back up which enables you to flash roms prior to MG4.
My S3 came with 4.1.2 installed already on MG4, that's the reason I am unable to go backwards. So I went forwards instead, I have now installed MG2, which is on the same structure and it seems to improve my reception etc
Let me know how it goes.
Mine's a Tiger Tops by the way