I kinda feel like that's totally on players. My simple argument would be, if you want to keep your oath, don't respec to avoid it.
I do see your point about it being an ongoing, more firm narrative but again, that's on the player. Any player's game isn't being run by a human DM to control what's going on, it's just code. There are some things the game can't do. Keeping players from cheesing the game to get the outcome they want is one of them.
Sorry for being negative here. I respect the time and effort you put into this post and do agree with it to a degree. The issue with
Shadowheart ditching her god but not losing her spells
does fall heavily into what your concerned about and it irked me somewhat that that wasn't addressed by the developers.