Get info should help for editing tags. Right click or use the File menu > Get Info.
The info dialog will show the file kind in the summary tab. You may find that certain tags can't be changed on certain file types, I think WAV/AIFF's don't allow artwork, you can use the 'Advanced menu > Create xxx version' to make one that is editable based on your current encoder settings (in the iTunes preferences).
You can also bulk edit tags if you select multiple items, which is good for setting album names or making the artist name the same case or spelling etc.
You should also consider asking iTunes to get artwork for the items that were on LP, many have been released on other formats. There are apps like Tagalicoius or Music Brainz Picard that can help getting the artwork & tags etc. Just have backups before making major library changes, the files get updated, so that may effect any other apps that use them too.
I'd also use the popup menu to reveal files in the Finder/ Windows explorer, check the permissions are correct, if you don't have editing permission you won't be able to edit the metadata. I'd select a specific song, find the file, copy it somewhere else, delete it in iTunes and then reimport the file into iTunes to see if it becomes editable, check the permissions again.
The things you describe don't seem normal, iTunes can do many of the things that seemed locked for you. Are you using iCloud or a network disk for the library?
Another question is what did you migrate from? Does that app save the metadata into the files? Ratings are usually stored by the app not the files so you normally have to be carefull with how you import. I think you can create playlists of the 1 star, 2 star, 3 start… songs & then import that playlist to iTunes then batch change the ratings via get info.