I think you'll be let in pretty close to 3:00 pm (probably plus or minus a few minutes). In terms of Toontown, it really depends on what you mean by "see most" of it...there are two attractions (Gadget's Go Coaster and Roger Rabbit's Car-Toon Spin), some character meet and greets (Mickey, Minnie), four interactive areas (Donald's Boat, Chip & Dale's Tree, Goofy's House, and the Town), one shop, and some other miscellaneous things. I don't think you'll be able to do the rides and character meet and greets in an hour, but you can probably get one of them done plus "tour" Toontown (aka look around).
For my family, we would choose to eat during the overlap period so that we don't have to take time out of MHP to have dinner.