More on Gasly: apparently he was told that he would be racing in Singapore by someone from Renault - which is curious because Toro Rosso use Ferrari power (for now). Gasly says that his reaction to this new was surprise, since it was the first time that he had heard it.
Wild theory time - Kvyat isn't leaving Toro Rosso; Sainz is. Sainz is in the most awkward position in Formula One, unable to break into a top team because Red Bull will hold onto Ricciardo and Verstappen for as long as possible. In order to move up, first he has to go sideways. With Sainz out, Kvyat fills the same role as Vergne: a (relatively) experienced barometer for Gasly's success.
Of course, if Sainz is leaving Toro Rosso for Renault, someone at Renault has to go; I can't imagine that he would sit a third of the season on the sidelines, especially since from Renault's perspective, getting him early would give them an opportunity to work with him. And Red Bull would certainly have the resources to buy Magnussen or Palmer out of their seat (especially as a show of good faith to Renault).