So after wondering about this very issue for the past (close to) two years, I finally have the answer.
The show they played last Friday in Atlantic City had a bit more stripped down stage arrangement. There was no curtain to unveil the big screen or bringing in of the big instruments after Witchy Woman; it was all there on stage before the show. During Saturday Night, I happened to catch Steuart Smith upstage left (behind Glenn) playing a mandolin and another backup musician (possibly Michael Thompson?) upstage right (behind Don) playing an accordion.
I finally solved it! To sum up, during WHTSN, there IS a mandolin AND accordion being played live behind the curtain that normally would have been there.
Michael Thompson is seen playing accordion on the F1 version of Tequila Sunrise (and I'm presuming he still does), and Glenn implies sometimes during the intros that he is able to play a multitude of instruments, so it is most probably him. Very audible just before the start of the second verse iirc.
On that point, does anyone else hear accordion in Doolin-Dalton? I've never been able to see Michael close-up during this song but I'm sure there's a hint of it on some bootleg videos (near the end over the humming)...