As far as a third B-29, there is Fertile Mertle which last flew in the early 80s. Kermit Weeks bought her, and as far as I know there are no plans to make her airworthy as of now.
I got to see Doc after it was pulled out of China Lake. Orange Empire R/R Museum was allowed to remove track going into the base when they pulled up the Jawbone branch going up Owens Valley. While we were working there over several months, Doc was towed to the Inyokern airport. To say he was in rough shape was an understatement! They guy who owned it traded the Navy a B-25 for it and eventually it ended up in Kansas. They are still one engine and about a million dollars away from finishing it. I wish them well.