I'm pretty sure there were SEVERAL basses in the song, at the same time.
This is true but again, see the Smile box sessionography for what made it to the record and what did not. We know exactly what session this footage is from, and the bassists on it are Carl on Fender and Lyle Ritz on String Bass. Pitman, I think, is on Danelectro 6-string bass.
Something else weird to me... is that really Al singing "gotta keep those lovin' good vibrations a-happening with her"? I thought that was Mike.
I always thought that was multiple BB's singing that line. (not in harmony)
Yes, that is a unison group vocal.