The Seanchan won't know who of the AS is BA or a Forsaken, so they theoretically have to collar everyone.

I think Rand's thoughts in TGS, ch.1, show that he would basically do anything to unite mankind and that he isn't very fond of AS. Agreeing to an attack on the WT on the premise that the Shadow controls it and has to be expelled from there before TG, seems even reasonable to me. After all, Rand also attacked other countries where Forsaken were.

But I actually wonder now, if the Shadow will tell the BA that the Seanchan attack is imminent and pull them away from the WT & the Rebels.

The attack of the Seanchan could be also another reason why Mesaana might not live under a certain identity in the WT anymore.