Great, so it works 100%?
I use uCON64 to fix checksums, but this looks like a simpler program:
http://segaretro.org/ESE_FixCheckSum
My guess is that the graphics are stored in the upper ROM and when the console enables it, the internal pull up resistors trick the console into reading all 1's or all black for graphics. This is merely speculation, But an interesting find none the less. Actually...now that I think about it, the SNES version of Phantasy Star uses two 16-bit roms on the original japanese game. Surprisingly the translators found that the english text was stored only one ONE of the ROMs, which meant making a repro of that only needed one ROM switched. Sorry for that aside.
In any case, I changed the region for my ps4 ROM around a couple times and it seems that the header does not effect the checksum. Oddly, the game has Brazil as the only region but I ran it on my USA-only BIOS and it runs perfectly. I even went as far as to change it to Europe and it still ran on a USA BIOS. The fact that you got sound and did not change the region says something about the game: Apparently it doesn't care what the region is.I have never come across a game like that.
Where did you edit checksum? It should be at the check-mark and not the X:
Spoiler warning: