I had a fair amount of heartburn over these edits at first, but ultimately they generally fit in with our title practices.
1. Animex 1200 prepend
We have a fair amount of precedent for including the series on the title (mostly with older anime releases).
https://vgmdb.net/search?q=series
You do make a good point though about Animex not showing up on the front cover. It is pretty prominent on the Obi though. As these are all reprints, I would bet that they didn't want to redesign the cover assets, so they used the parent album's cover with minimal changes. I know we can't infer their thought process with any accuracy, and there are at least a couple of ANIMEX albums where they moved titles around, or made other changes. In the end, I am not so concerned about this because this is one series of reprints, and the parent albums are still in the catalog alphabetically.
2. ヒット曲集
This one is tricky. The phrase 曲集 translates to "collection (of musical pieces); studies; album; suite; ED" on WWJDICT. One rule we usually follow is to use any English text on the Front Cover in the display titles. Unfortunately, this has been rendered in English in many different ways on albums we have here:
All Sounds of....
https://vgmdb.net/album/199
(全)曲集 (Complete) Song Box
https://vgmdb.net/album/7824
Music Collection
https://vgmdb.net/album/68275
I think I am more in agreement with you on it being Hit Song Collection rather than Collection, maybe Hit Music Collection for non vocals.
3. Ongakushuu in Display Title
I did a quick search. There are 925 albums with 音楽集 in the title. Of them, only 354 of them have Ongakushuu in the display title (and some of those are recent edits by Anisonfan.) I know that in the past, as you said, we always translated any part of the title that described the album rather than the contents, so stuff like Music Collection, etc. I prefer that way myself, but there has been a slow movement away from doing this for years. I guess I don't have a good answer for this one right now.