Some final code requests (and screenshots)

That's a pretty blatant oversight indeed!

Simply put, those 'flags' are 'event bits' and are used for setting and clearing various event-related things, to control what happens or appears at certain points in the game, or what have you.

Assuming you'd want Relm to be there in the cave, a fix could involve using a different/custom/unused event bit instead that would basically do what the 'left Shadow behind' bit did in terms of putting Relm into the cave, but it would have nothing to do with Shadow at all so it doesn't effect him, and then in-turn update the Owzer mansion and Veldt cave entrance event and/or memory bit stuff to the new bit.

I think that'd be about it anyways.

The only thing I'm concerned about is that Leo is technically Shadow. I think Fedorajoe just swapped out the graphics and names. If an event flag is used for Shadow, it might effect Leo at the colosseum.  However, I am unsure as to how Fedorajoe made it so that Leo can be obtained immediately, while at the same time, so can Relm (in the cave).  I'm not sure how he set it up.