No, it's not always him in the suit. Sometimes it's actually John Wayne's grandson, Brendan Wayne, and there's a second stand-in actor as well. I'd be willing to bet that yes, all of his dialogue is probably ADR.
The comments I've read from Favreau and Hamill indicate that not only did Hamill do the voiceover, he was actually on-set and acted out the last scenes of taking Grogu from Din Djarin and walking to the elevator. They used a combination of de-aging and face replacement tech to pull everything off.
AND, for me believe it or not, his belt and belt-buckle. What a great season and an absolutely AMAZING finale! I love what they've done with this show!
Yeah, I totally forgot to mention Biehn. I thought he was OK, but neither his character or his performance were anything special. And, as Johnny mentioned, he's one and done, which is a shame. Still, it was good to see him on-screen again.
Disney has announced some release dates, including for the second season of The Mandalorian, which hits Disney+ in October:
https://www.polygon.com/disney-plus/2020/2/4/21123154/release-dates-mandalorian-season-2-wandavision-falcon-winter-soldier-mandalorian
Totally agree. Vader is the most iconic character in Star Wars, and one of the most iconic film characters in movie history. HE is the face of the franchise as far as I'm concerned. As long as I ignore what Lucas did to him in the prequels that is. ;)
The third episode was GREAT! I really like how this show is unfolding, but the short runtimes are killin' me. This is the third time I've found myself wanting more, more, more once the credits start rolling. Awesome stuff!