1. You actually can go unlock it, just visit the places where Chimera and Hydra were and there will be a thing to interact with (marked Destination iirc) which opens the entrance for them.
2. Since they're no longer in the actual world, they're likely to be DF queue'd as often as HM Primals are. Usually people will queue if they can't find a group, but it's not really hard to find a group for stuff like Chimera and Hydra.