FINALLY getting my head around MP3tag...
cleaning up tags worked for me (mostly) as follows...
I had replaygain tags I didn't want but DIDN'T use mp3gain to alter files' data
I had ape tags and id3v2 tags on some files
to clean up id3v2 tags
select all, write new tags, delete tags
change options to
select all, REFRESH, write new tags, delete tags, refresh
Everything looks cool - it's the deleting of the entire id3v2 tag and reconstructing it that makes the garbage go away. messing with "advanced tags" didn't do it.
the basic tags (the ones I wanted) move nicely from tag to tag. Everything else gets discarded. I can always do replaygain over again.
What I can't do without scripting is to MIX tags from id3v2 and ape into one - oh, well. Thankfully there's not a lot of those, once I gave up on saving replaygain (which I gave up upon finding two or three different ones created by different apps...)
which leads to my next post... in feature requests