1) Remap is always better than a plugin device. Personally wouldn't bother on a non-turbo engine unless there's a real gain to be had.
2) Not sure about springs, Eibach are popular but my first change would be getting rid of runflat tyres, that is the biggest improvement to the car.
3) Water ingress is a known issue with several causes. Water can get in through a vent in the battery compartment, from perished seals on the boot lid or through the rear light clusters if they are incorrectly seated.