Challenge yourself to learn how hex editors work and the layout of the save file -- set the byte width to 0x1000, then you can corrupt the most recent save blocks so the game loads your backup instead.
Save data structure (Generation III) - Bulbapedia, the community-driven Pokémon encyclopedia
PKHeX does not allow loading of the backup portion, only the most recent part of the save.
I guess you could debug the program to tell it to load the not-latest portion, but that isn't something worth explaining in detail.