I accidentally put my 3 villager plots on an island different from the one I put the bridge on as there was no space. Will my villagers be able to get to their houses without a bridge?
Yes, they will be able to reach their homes no matter where you put them. I put mine across a river and up a cliff (so I need both the vaulting pole and the ladder if I want to visit them) and they've moved in and are sometimes home or out wandering and it hasn't seemed to have affected them overmuch. I suspect the game accounts for this, and lets villagers move freely even if you have to use tools to get somewhere.