To the first question, as far as I am aware no. Scaling shouldn't effect Force in any manner. As for the second, it should be effected, as pyromancy and sorcery are to my knowledge.
It is, it goes from being almost instant to nearly instant. Its almost unnoticeable. I believe the dex reduces casting times by a percent not a constant. But yes it does effect the casting time.
I had no idea on the scaling. On the speed of casting I figured Dex would increase it, but I watched the following video and at 1:49 it looks like Dex didn't effect the casting time of Poison Mist. I figured some spells might not be effected...