Well, if they've been good they go to heaven. Failing that, they either sink, decompose, bloat and float, then sink again, or for some whales like right whales they float immediately. Either way, they get eaten by small critters. Nope - only adult males. Not sure what it's used for - probably males fight each other with the tusks.
Most large whales do, but the pattern varies among species.
No - there are no reliable reports of this. Humpbacks are the easiest because they have a very variable pattern of black and white markings on the underside. Male beaked whales have barnacles on their teeth sometimes. Great question, don't know the answer.