If a villager asks to move in, then there is enough room for them and it won't mess up any other villagers moving in. Once you run out of spots, no villager will request to move in. Found that out the hard way when I had an adorable villager camping in my town, I really wanted to get her to move in, but after talking to her for ten minutes straight, I finally figured out my town was full and that was why the move-in prompt never appeared. Also, if Benjamin is the camp spot or in another town, it usually takes 2-3days before their lot appears in your town after asking them to move-in, so Marcel should already be moved in before Benjamin's lot shows up.
Oh and if for some reason it doesn't work out and you can't get Benjamin, I have him in my town and don't really want him. I can let you know when gives me a move-out date. I like Benjamin and he's a nice villager (love the music it plays when you talk to him), but I'm trying to open up a few spots in my town, so I can get my dream villagers when/if they camp in my town or find another player who has them. I time travel in my game, so I usually have at least 2 villagers who move out every week.
Anyway, hope that info helps! : )