As for why not kill the countess as well?
They had no offspring, and the law would dictate that instead of distant relatives inheriting the lands, it would be Alastair Greenvale himself who'd gain the lands. Quinlan did not want that to happen.
Furthermore, it was their goal to have as many useful puppets as possible. Killing low-level people like her did him no good, so the choice was simple.
Lastly… Indeed, this had been planned. The whole sequence played out according to Quinlan's design, save for him underestimating just how devastating Morgana's spells were.
He'd been notified of the development in the heart of Greenvale, namely Kaede and the others coming to back Alastair up.
He knew they were searching for his location, told by not only the Greenvale twins but also Cassandra as well, Morgana's royal guard, whom he subjugated. She and the other guards were already dispatched to investigate his previous targets, as the woman did not trust Greenvale investigators.
