Additionally, Bilbo never sought to DESTROY the ring, thus the Ring had no reason to protect itself from Bilbo and therefore had less of a grip on him.

What would happen if Bilbo didn't give up the ring?

If, in the end, Bilbo could not or would not give up the Ring, Sauron would continue with his siege on ME with Rivendell and Lorien be like islands in the midst of war that would eventually be overcome.

Is Bilbo the only one to willingly give up the ring?

Ring-bearers

4867 years and was held by nine people, five of which were Hobbits. ... Tom Bombadil wore the momentarily Ring on September 26, 3018 but was apparently unaffected by it. Of those who ever wore it, only Samwise Gamgee, Bilbo Baggins, Frodo Baggins and Tom Bombadil gave it up willingly.