I have actually never talked to them, but I follow them on instagram: Hauntedbymel is the one that got all these songs and they're the one selling them to everyone else (I think they had an insta account but they deleted it), @vintagemuu (leaked SFF and Half Hearted), @CryBabyBoiiiii (he leaked Schizo) and @littlebodybighatter (leaked Haunted, but I think they're done leaking now), all answer to questions on insta stories such as "what will leak next/what's left to leak/when will you leak [x]". As far as I'm aware all 4 of them are in possession of 99CS and Unhappy Meal but refuse to leak them until the album comes out. So that's where I get most of my info from. Also on the Melanie Martinez wikia, there's also a lot of drama going on in the comments of the pages for the unleaked songs.
And yeah there's 3 snippets leaked of UM as of now:
https://www.youtube.com/watch?v=TVP1qQapeNY