Though Cassian Andor was certainly part of a cell close to Mon Mothma before that.
I'm not sure what you base that on. Cassian was from an outer rim world (implied to have been allied with the Seperatists IIRC) not from any cell local to Chandrilla. The only connection with Mon Mothma is what we see in Rogue One; he's a Fulcrum agent operating out of the Massassi unit. If anything, any association he'd have with Alliance brass would have been with either Draven, Dodonna, Cracken or
maybe Ahsoka herself.
Remember, what became Alliance Intelligence was started by Bail's end of the organisation (mostly beginning with Ahsoka taking over his networks a year into the Empire's reign) whereas Mon Mothma was more the public face, and even then only outing herself just a few years prior.
Indeed It seems like Bail was the one organising the cells behind the scenes so that she could rally them with her declaration of the Alliance. We don't really have a sense yet of what she did prior to Dantooine; not to say that she wasn't a HUGE part of the Aliiance behind the scenes; most likely helping Bail hide the money trail and drawing any suspicious ISB might have away from him and squarely on her.
I suppose what I'm getting at is that Mon seems to have been the velvet glove, while Bail was the (very sneaky) iron hand. So I don't see her as being close to any particular cell, much less whatever Cassian's was before Yavin.
Where she went after that isn't fully known, but she definitely regrouped with the combined Alliance Fleet for Endor.
As per the comics; she commanded a few capital ships for the Alliance prior to Hoth, one of which was a mobile flight academy for Alliance pilots (an old Federation droid control ship no less) and another was one of the first Mon Cal cruisers to roll off the assembly line. By Endor though she seems to have been back on the Ghost, probably leading a wing of fighter squadrons like Lando in the Falcon, while Rex was down on the surface with Han's commando team.
I got the impression that of the remaining Spectre crew, is was mostly just Sabine who remained on as the protector of Lothal while Hera, Rex and Zeb went back to serve in the larger rebellion.
