Yes, the dolphin striker, which goes through the beam and is welded to the mast step, is free to move up and down.
The beam itself inserts into a socket laminated into the hull. I've attached a photo showing how far the socket goes into the boat. It is integral to the bulkhead.
Yeah, I think the stainless plate performs a bunch of functions; wear, centering, and strengthening.
The rope is actually a stainless wire with a turnbuckle. According to the importer, Pilkington built the boat with this to stop the beam from rotating when "under extreme load." That's never happened and we're really had the boat in some hairy stuff. I've since replaced it with dyneema. It would take some load I suppose, since the mast crossbeam is elliptical in profile, so if it rotated you could imagine what would be happening to the hull!