Good news. GameStop offered to ship to my house a replacement copy of MGS2! I didn't even have to argue or bargain with them, they just took one look at the PS2 and one look at the disc and were like "okay we'll ship you one from our warehouse"
I know all the horror stories so I had all these logical arguments planned ahead and I'm glad I didn't have to use them. Thank you GameStop for not boning your customers over for once.
Here's to hoping the replacement copy they send isn't nearly as scratched as the copy I showed to them.