1) Are multi-hit moves decreased in 2v2 by 20% of the original BP? Or some other percentage?
2) Does Will-o-Wisp have an increasing amount of burn damage? Ex. A Toxic, but with fire instead of poison.
3) With Swift Swim, is the Speed stat doubled in rain?
Probably not, no, yes.
how much speed should a gengar have if it is holding a choice scarf?
The Gengar's speed would be x1.5.