Did you ever figure out what was going on with One Way or Another and Wonderwall?
Not yet, sorry. Honestly those slipped my mind and my queue. I'll do some investigating soon.
Interesting project... I have a couple of dumb questions
Since the additions are only in the _plus.mid file, I suppose that adding keys won't make their audio controlled by the player, right?
Actually keys will control keys audio... more or less. Almost every song in the game, including every song with an available keys upgrade, has a "fifth" track for the background audio. Each song's updated DTA has code that links that audio to the keys chart. For some songs, it's a perfect solution. For other songs it's not quite perfect, since missing a keys note will also cut out background vocals, extra instruments, and anything else in the background track. But it's better than nothing, I think.
Then also obviously, these updates require to have first exported songs into RB3?
Yes, any song you wish to upgrade needs to be playable in RB3, either purchased as DLC in RB3 or prior (not purchased in RB4), or exported from the earlier discs.