The bassline (Ostinato if you wanna be technical) runs on a single note throughout the whole thing, so that really determines the key signature. Since it's at a constant Bb, i can guarantee it's in Bb Minor. There is a few minor thirds in there too, which also helps.
Hey, splicing songs together and whatnot is inherently tough stuff, don't feel too bad about struggling with it. From experience, it's typically easier when you actually get to choose the subject songs XD. The attempt itself is an accomplishment.