that Tommy wants to become president, taking Rusty out, simply doesn't make sense. his DIKs rely solely on Rusty's money, without MC's sermon they would have already folded as a fraternity.
Quinn's help doesn't make sense either, it's obvious that the money Quinn makes isn't enough for her as it is and why would she help Tommy? DIKs aren't that central to her business (HOTs is, but not Quinn's)
but more importantly Tommy as VP has all the freedom that he would lose as president; for Cathy to get Rusty in trouble precisely because he is president, why would Tommy want that exposure?
in the matters he cares about Rusty doesn't stick his nose in (in a slightly less cheating way than between Quinn and Sage).
a completely random culprit can't be ruled out either, but it would be a disappointing solution.