It definitely seems to be Hayden in the Vader suit. He's no longer as tall or broad as David Prowse.
The show isn't great, but it is entertaining enough to keep watching. Ewan is great, even if his character always was (and still is) boring. But occasionally there are fun character moments, so Obi Wan definitely is slightly more interesting than he was in the prequels. The sets and costumes look great. The effects are good, but occasionally there's scenes that come across as rather low budget. Such as that bizarre laser barrier that looked small enough to hop over. The action varies from fun, to poorly choreographed. And the plot, while simple, is servicable. At least there is one, and despite it being riddled with tiny plot holes, for the most part it is alright. And the kid playing Leia is really well cast.
For all its minor faults, I'll keep watching. I don't really care if Reva's lightsaber is a practical weapon either. That just seems like nitpicking to me. It's fantasy.