#2
You probably will want to worry HOW to take it off instead of getting it on.

Lighter Fluids.
#3
As long as it has a good clear coat to cling too, it will stick. Most decals I've seen are something along the lines of waterslides, but car decals I'm not too sure about. My thing is, as posted above, if it has an adhesive on it, it may leave a wierd glue spot if you ever try to take it off.
#4
Car finishes and guitar finishes are extremely similar, so I'd say it won't be a problem. May I ask what decal it is?