I really like mp3tag a lot and use it every day. I would like to know if there is a possibility while using web source to be able to grab the artist from Amazon (if available) and not only save to %artist% field, but also %albumartist% field.

I'm not 100% percent sure that if Amazon erases the %totaltracks% field for m4a/aac audio because those field are separate unlike mp3 files. I know mp3 files have %totaltracks% removed when tagging from Amazon. I would like to know if there could be a fix to remove %totaltracks% field from mp4 and replace it with only the %track%.I was also hoping when using Amazon web service that since mp3tag doesn't deal with multi-disc releases and instead increments the tracks in sequence that it could remove %discnumber% and %disctotal% from the tags of an AAC file.

Mp3tag v2.50 supports total tracks/discs on Amazon.
And scripts can't remove tag fields, they can only overwrite them with a different value.

You can use the action tool to achieve all your desired changes after the Amazon tagging.