I found A,B,D,F,J and now it seems obvious to me why I was missing E. I'm on the same boat with you on C though, no clue what triggers it (edit2: I found out, it requires having the same low stat requirement as Ending D, but with something extra)
It appears that ABCD are Endings, while EFJ are Endings... of another kind (assuming that J means joke ending, the way to trigger it seems like one too)
Edit: J does not mean joke ending, just discovered Ending I... How many endings are there???
Edit2: Missing possibly Ending G and H, dunno if there's more