A) Typically Disney releases their deals about 3 months before, one season at a time. You should book as soon as you know when you’re going though if your reservation is cancelable; that way you can guarantee your reservation but get a better deal later if it comes along. I understand they will apply discounts later if you call them after seeing a deal. I highly recommend you scour this MouseSavers page about resort hotel discounts:
B) Are you referring to the Disney hotels or good neighbor hotels? Grand Californian is far and away the best quality, and most convenient to the parks. You can try renting a unit at the DVC there and get a bit of a discount over rack rates. Paradise Pier is the best value. Disneyland Hotel is nicer than PP but a bit further away. Still an excellent option. You can get better rates across the street on Harbor Blvd at the good neighbor hotels.
C) I have not seen a specific date released for Marvel opening other than “2020.” They have a big incentive to have it ready for summer crowds, so there is a chance it will be open by August. I’m skeptical though - I wouldn’t count on it.