@SirBlackMage, your post made me curious enough about the issue to poke around using FEBuilderGBA and compare this hack with an unmodified copy of the game, and I figured out what happened: Apparently in the process of reverting the changes to the final chapter the event that causes Nergal's door to open had been deleted by accident. I've made my own version of the patch that should fix it, or you can do it yourself by simply copying the event data starting at address 0xCBF930 from the vanilla US release (if you use FEBuilderGBA to do so be careful, I found that it was easy to accidentally delete the event after the one you're adding by mistake depending on how you do so).
I should note that I haven't actually playtested this yet to confirm the door opens now, but considering that FEBuilderGBA was warning me about an error when I opened it with the original version of the patch and that the error went away when I copied over the event data from the original game I feel fairly confident that it should be fixed.
FE7 Gameplay Reversion v1.2.ups
