Thanks for your interest in helping, but to be clear, I don't need the API key. I am registered developer with Apple, and can make my own keys. If I had the time, I would probably just make my own app to do the tagging. But MP3Tag does a very good job and it saves me a lot of time.
There are several ways to access the Apple Music API in an official way now, but for the moment none are possible via the Web Source framework. One option which recently available is using javascript, which is not supported, the other way is to authorize via http headers when requesting Apple servers, which also is not supported. Apple have expanded the ways to use the API lately so maybe there will be other ways in the future.
As for the script itself and the metadata, I make the script for my own needs, so any manipulation of the various fields I require will also trickle down to the script I upload. I do try to adapt it if someone asks, if I can make it within my needs, or point to how to change it themselves. One thing is the album and song titles, in Apple Music and iTunes, variants and extra information is tagged on to the title. I put this information into comment instead, as it makes it easier for me when I play back in my player. But as you have shown it will always be a compromise.
I wish there were more possibilities to have some "preferences" options for scripts. Even though the script is quite simple, it requires some basic understanding of programming, if you want to adapt it to your own needs.
I could as many other web source authors have done, make a lot of different variants, but then I don't think I want to jump into the rabbit hole of managing so many variants.