What happens if a player were to die before entering the Medium, but were somehow revived before entering as well? Is that possible? More of a opinion question I guess since its never happened (the closest instances being with Sollux and the Alpha kids).

That did happen with Roxy and Dirk.  In that case they could use the teleporters on Prospit and Derse to get back to Earth.  The Derse duo ended up entering with Jake.  How other players would pull that off is unknown, they’d probably have to hitch a ride home.