I've been wondering that too...although I noticed him many a year ago.
He's my favorite ROTJ pilot (sorry Wedge). "She's gonna blow!"
I have no idea who played him. I'm not even sure the character has an official name. I'd love an action figure of him though.
As for the dignataries...I don't think any of the actors have been identified..and it's possible they may be deceased. There are character names for some of the digniaries. Sim Aloo is the tall one with the turban-like hat...the same one the vintage figure was modeled after. I forget the others off hand right now.
Now for some extra trivia:
Not to be confused with the ROTJ dignitaries is Sate Pestage. Pestage was an expanded universe character created by Lucasfilm/Kenner in 1984-85. He was supposed to be a part of the new toy and possibly a cartoon series back in 85. It never happened. However..the character Sate Pestage (Palpatine's Grand Vizier ...his head dignitary, so to speak) was used in West End Games sourcebook material and again in the 1990s in the X-Wing comics by Michael Stackpole. Since Pestage was supposed to be running many of the daily operations of the empire, I suppose he was the equavalent of Mas Amedda as far as ranking seniority.