Certainly leave the undercoat on if you're refinishing it anyway. I've seen someone recently finish a guitar using magic marble paint on another forum - however it did take him several attempts to get it right, and levelling it out afterwards wasn't easy. (Magic marble is a paint that you float on water and then dip the guitar body into - you'll need a big plastic bin, enough to submerge the whole body if you're going that route)
I'm not sure if I'd use gorilla glue anywhere. Afaik it expands in contact with water, including natural humidity.
As far as removing the existing finish, you shouldn't need to. Just sand it all to provide a decent key