First off, the movie has been out for like 2 weeks, do we really need spoiler posts anymore?
As for Thanos's motivations. They didn't put Mistress Death in the movie because it would take everything to the highest levels in the Marvel Universe. Simply put, the movie is not big enough for those type of characters and nobody but the comic readers would be able to understand.
Just about every character in the movie is so incredibly insignificant if you factor in the godly beings that Thanos fights. (Galactus, The Stranger, Order, Chaos, Love, Hate, Death, Mephisto, Eon, Celestials and Eternity itself.) Do you really think Thanos would give 2 shits about getting punched in the face by Captain America when he can shrug off a blast of Cosmic Power from Galactus? I guess what I am saying is that the godly beings (like Mistress Death) are simply too big for the movie screen hence why the movie takes a different path from the comics.