What exactly are the rules for Congress people who don't show up to work and basically go absent without leave? Under such laws, who has a legal duty to follow up on the situation, cut pay if the big lump is not working, and eventually elevate the situation to termination, which creates an empty seat and an election?

Also really important, since Fetterman has not been attending Congress, how has he been putting his name/vote on various issues? This is some kind of fraud, if his office staff can do this while he is incapacitated/dead.