A 23
Becoming a lycanthrope is very much like multiclassing as an animal and gaining the appropriate Hit Dice.
You can only identify the Humanoid aspects of the creature with Knowlege (local). One of those aspects is that the creature has the shapechanger subtype. If you exceed the minimum DC required for success on the check by enough multiples of 5, you may discover that.

A 24 No.

The Heal skill may be used to treat the effects of diseases, but AFAIK cannot be used to identify specific diseases affecting a creature.