Thanks for the link, evandixon. Unfortunately, it's not quite what I'm looking for, though I think the code itself may help somehow. My game crashes as soon as the save file loads (and the journal entry is flipped, so basically when it tries to load the map) so there's no opportunity to save. However, the code did at least do something: when I held Select and booted the game, the game played the first note of Twinleaf Town then froze instead of Jubilife City's. On another non-corrupted Diamond, when I tried booting the game holding Select, it had some interesting effects.
-When initially booting the game, it put me in a Twinleaf town void (the black screen like when you run out of bounds).
-I couldn't move until opening and closing the menu, but then I could run to seemingly real places like Route 201 and Sandgem Town by following what would have been the correct path.
-Few obstacles were present for the character, similar to when you reach Fake Sinnoh.
-At any time, I could open the menu and Fly to the real Twinleaf Town with no problems.
-When loading the game from having saved in Twinleaf Town, the game loaded the graphics of Twinleaf Town, but spit me out at a different coordinates than what I saved at. The boundaries did not match what Twinleaf Town's boundaries should have been. It appeared like the map itself had been shifted, but not the graphics. Again, I could easily Fly to Twinleaf Town and everything would act like a normal game would.
Based on these observations, I think that the problem lies within the map coordinates themselves where I saved. I attempted to save holding Select while migrating Pokemon over, but it must not save the map data when it migrates the Pokemon, because nothing changed from my original problem when booting the game normally. It looks to me like I need to change the map coordinates themselves upon startup, probably using this data, but again, I don't know how to write Action Replay codes as of right now, though I'm learning. If anyone has any more info, or knows of something that could help, all suggestions are helpful.