So in regards to the fact that it would either A). Have players not understand their role or B). Have the round take too long:
Give the original person a 30 second timer. If they have not killed a single person in 30 seconds, a random person is chosen to also become a prop.
If people are camping really hard (which likely would be a legit strategy) and no kills are occurring; make the prop teleport to a vicinity of players who are not dying. If the prop cannot be within that vicinity (or if its just preferable this way) teleport the player (or players) to a random location on the map. This doesn't have to be a static gamemode. It could be that every 30-60 seconds players get randomly teleported and need to find somewhere to be safe from props.
I also suggested it could be an "infected" gamemode so to speak, where once a person dies they also become a prop. This would very quickly cause a chain reaction and the round to accelerate.
TLDR: There's really no reason why this proposed gamemode has to be a camp fest or take a long time whenever it occurs.
I mean hell, even the Reapers had a teleport functionality for when people were camping in hard to get to places. Garry's Mod spaghetti code permitting, I don't see any reason we couldn't implement something easier and better for props to teleport to a room or nearby building, especially near people.