I just realized I wasn't clear enough on the matter of the timeline. Obviously Siuan's team was sent in TSR, and the decision to send Memara was made in the TFOH prologue (which is incidentally not long before Siuan takes control of the Blue Ajah network temporarily along with regaining control of her Amyrlin's network, though she claims to have no knowledge of the Borderlands in LOC - whether that is true or not is a different story, as Siuan probably kept her most trusted agents for herself - we have indications of that in ACOS, though of course she passed on her best reports to Egwene, and Egwene only). Taim publicly declared for Rand at the beginning of LOC, and it was common knowledge in Salidar by the end of the book at the latest. We see the Borderland rulers in the prologue of TPOD, and they'd of course been on the march for some time, and incidentally, they met in the Black Hills, which is where Taim escaped in the first place.

I imagine that, if these Aes Sedai were indeed Siuan's, that the bulk of them - seven of the twelve - initially approached Paitar because of his known relationship with Aes Sedai - Ethenielle seems less trusting of Aes Sedai, and I doubt this detail was overlooked in the choosing. I'd say that Coladara and the others probably felt each other out and realized that their goals were similar. It may even be that one or more of the seven actually knew Coladara in a somewhat friendly manner; out of twelve they should have had at least one or two. Paitar might have even been aware that Kiruna had gone to Salidar to join the rebels, though I doubt he was aware of her trek to the Waste and her swearing to Rand.

Five went to Tenobia because more might have been too many for Tenobia (though five are a delegation worthy of a ruler), and because of their original mission to recapture Taim - they should have known that Bashere was tracking him, at least - and when they discovered that there was an emissary of Elaida in the palace, they decided to stay in the city instead, or perhaps even outside of it (though this seems unlikely - unexplained Aes Sedai in cities seem relatively common in recent days, and cities are more convenient). From there they learned of the preparations to march south - well hidden by Tenobia, of course, but not well-hidden enough to escape the notice of informed and intelligent Aes Sedai. That is probably they caught up with Tenobia just as she was leaving - accidentally, of course - and I doubt they missed the fact that Memara had been left behind. The time of departure would be the most difficult time to hide said departure.

They might have had contact with Siuan through Blue agents in the Borderlands, whether they knew it was her or not, or they might not have. If Siuan did manage to pass a message, it might have been something vague like 'keep tabs on the rulers' and she might not have ever received word from any of them. In any case I find it to be a plausible scenario mostly because RJ was a recursive writer and he liked to incorporate already-introduced details into the story as much as possible. The mystery of the Borderlanders as a red herring is sort of a disappointment in some ways, but a detail like this tying it all the way back to TSR - perhaps a brief POV in AMOL - has the potential to make it less so.