I didn't mean to argue with the way you work or your edits the but simply tried to give answer to your question about 3 tracks.
Sticking to booklet is good, going further means no harm IMHO.
To me, anybody looking for quick answer to whom is singer/songwriter/lyricist for a specific track should be able to find the answer directly in notes. Many times, edit sources are lost (pictures or webpages no more available after a few years).
Now where to put these uncredited credits, it's up to you, your edits; no guideline for such things. Sometimes, it's directly next to credits (
http://vgmdb.net/album/36847 )sometimes as an extra footnote (
http://vgmdb.net/album/23876).