Having used multiple drummers applies to several artists, notably Zappa, McCartney, Steely Dan (I know I'm missing some...) and there are various reasons for it. Sometimes the drummer has other commitments and another needs to be brought in. Sometimes the drummer gets uppity and demands more money/treatment than was agreed-to. Sometimes drummers have or develop problems that weren't evident when they were hired, and they are cut loose. Sometimes 'management' wants to save money and is always on the lookout for players who'll work for less (and there are plenty of those!) Some simply become hard to get along with and last only one tour. In Zappa's case, he was just constantly looking for better players and held auditions on a regular basis.
And sometimes a drummer leaves on their own because there are more important things than playing, such as their children.
Using the artist's name on a resume/credits is completely legit, if the drummer actually worked for that artist. I don't know Aaron, but if he recorded with Swift, even just one track, he's welcome to list her. What's uncool is someone listing an artist who they might have opened for, even though they didn't actually work for that artist. They expect the reader to infer that they were hired by the artist, and that's a feather in their cap, bogus as it is.
Bermuda