The seasons in the northern hemisphere are opposite with those of the south. While in one was fall, in the other it was spring.
And the subsequent dust winter lasted more than 1 season cycle.
Based on the above I suspect there is no big difference in the outcome by just changing the season when the impact happens.
Subduction of the plate carrying the landmasses.
Subduction takes place at the boundaries of tectonic plates. One plate slides beneath the other. If the plate that is going down is carrying a landmass, that landmass will go down with the plate.
An example: the Torres islands are sinking below the ocean, because the plate they are on is carrying them down....