it's a "neat" theory, but it won't work for several reasons:-

1) Inclined Moons are unlikely, and the tidal flexing you describe would rapidly circularise its orbit.
2) Geothermal heat cannot warm a planet's surface and atnmosphere appreciably.
3) Although a collisional origin of the Northern plains IS the most likely (and I agree with it), it ias likely to have been in the final stages of the assembly of Mars, not some time later, and from a non-captured impactor.
4) In fact, we can DATE the age of the northern plains basement and find that it's far older than any evidence of surface water.

So maybe Mars had this history, but it was 4 billion years ago. I have no problem with early Mars being warm and wet - ephemerally. When the Early Earth had a steam atmosphere, Early Mars had liquid water. No problem...