Mabel
I think what you are describing here is the current state of the art when it comes to metadata, editors, and the ability to keep things from getting sideways. I personally, and I'm just one voice in the jungle, don't know of a tool that will go out and get metadata from a source, but will only place entries in to your selection where no entry exists, leaving the current data unchanged.
I am a genealogist. There is a function in the editors there that will gather data from a source and place it in a pane (or window) that sits side by side with a pane that contains your existing record. This lets you do a side by side, line by line examination of the data fields. It also lets you grab a field from the new source and drop it in selectively to your local record and only change that field. This sounds like what you may be looking for. Keep what you already have entered, but add to or change selective elements under your direct control.
For music metadata, I don't think we have that. If we do, I'd be interested. Here's a business opportunity!
Find a 5th grader, tell him/her what you want and have them code it up.