Not a long time. A few weeks. Suroth sent them west at the beginning of KOD, and right before Malden they were back in Altara, after Galgan had them recalled. The returned scouts spotted Galad at the end of KOD, and Tylee could have to'raken again to send the Shaido captives to Ebou Dar. Northern Altara and the passes to Murandy are crawling with Seanchan armies and patrols. It's hardly surprising that rumours of the flying beasts have reached Caemlyn, and the Seanchan's hatred of AS is well-known.

The beasties assigned to the east had been back in the east for quite some time when Tuon launched the attack on the WT.

That the Mat scene could take place after the battle at Tar Valon and Egwene's reunification doesn't make any kind of sense.

Calculate it any way you want, this would contradict not only the timeline of TGS that we can figure it out, but also everything Brandon has said not only about the timeine, but also about the structure of TOM... with Perrin having a lot of catching up to do, and Mat a bit less but still some catching up to do. You have Mat already ahead of Egwene and Rand by chapter 8. This doesn't make any sense, and even less when you consider there's so much "recap" and bringing up to speed in this chapter that it almost certainly is the first Mat chapter in TOM.

For Mat to be in Caemlyn for much less than 30 days, and this to coincide with the reunification of the Tower, he would have had to spend many weeks between Hinderstap and meeting Verin...

I mean... we know exactly when Mat's first scene in TGS happened, to the day. We also know to the exact day when Tuon reached Ebou Dar. Brandon stated there were weeks between her meeting with Rand and the attack on the Tower. He also stated clearly that TGS covered about or slightly over 3 months, and that Mat reached Caemlyn 30 days before the climaxes of TGS (also that Egwene's and Rand's endings coincided to the day...), and in chapter 8 Mat mentions he's weeks away from the end of those 30 days... Do the math. Unless Brandon and Maria did a MASSIVE screw-up of the timeline, the attack of the Seanchan on the Tower is weeks away from that Mat scene.

That wouldn't be a first that rumours are used for ironic foreshadowing in WOT by being all too close that what will happen. Jordan used that device a few times (incl. at the end of TPOD, IRRC - foreshadowing the reunification and the Seanchan attack, and stuff that hasn't yet come about war between the Asha'man)

Had the Seanchan attack happened, there would be a lot more than just a rumour floating in Caemlyn. Pigeons from Tar Valon would have flown to most cities with the specifics, especially to Caemlyn. There's a big hole in the White Tower, for crying out loud... Elayne would already be on the war path, preparing for the eventuality of a similar attack on the BT or on Caemlyn. Everybody was counting on the Seanchan avancing slowly... Tar Valon changes all that, and it won't escape Elayne that the Seanchan may well have stolen the secret of Travelling if they attacked the Tower. Keep in mind that with the reunification and her freedom, Egwene has no motive not to contact Elayne.

Nah... clearly those rumours are just rumours. I can't believe Brandon and Maria could mess it up so massively. That's no little mistake, it would screw up the whole continuity since KOD!!

Further elements that suggest you're wrong is that no rumour concerns a large group of AS arriving in the city (the rebel envoys). On the timeline as described by Brandon, they should arrive soon, but they shouldn't be there yet.