Can you please confirm that a garage floor/slab that is below the DFE must have breakaway walls on all sides of the enclosure area? Also, does this mean that the living/habitable area above the garage must be supported on piles, piers, or columns?

Confirmed. Breakaway walls are not a requirement on open foundations. A garage floor/slab below the BFE can remain open, be enclosed by insect screening or lattice work, or enclosed by breakaway walls. Also, since breakaway walls are not structural, the living area above the garage will have to be supported by structural piles/piers/columns.