I would do one or the other.Doing both will add weight and unnecessary extra strength. Plywood is pretty tough and tape at the joints and seal coats on the plywood should last forever
Yes, but the cockpit sole will suffer from abrasion (sandy heels) pretty quickly. So if the inside of the hull isn't glassed completely, it would probably be a good idea to glass that area.
BTW, glassing both sides does make the boat stronger - the plywood is just a very strong core for the glass, IMO.
And some folks glass neither side of the plywood.....
You're going to coat the surfaces with epoxy, probably, and light glass is an excellent 'depth gauge' for the epoxy coating. You don't need much in the way of fill coats on the inside of the deck, on the hull inside a fill coat is a bit of insurance against pinholes, but it needn't be heavy.