Since the 50th has begun the onsite hotels have been selling out further in advance. Thats not to say that they wont release some rooms before that time. Often they are holding rooms for conventions, and find they have more held then what they need. But, honestly, trying to book a month in advance you shouldnt be terribly surprised there isnt room available. Especially considering you are looking at the Thur / Friday of Memorial weekend.
We booked our trip for last Oct in April. At that time I still couldnt get what I wanted. I wanted a regular room at the DLH, with the 4th night free promo. There was nothing available. I ended up with a conc. room at the PP.
As for how busy it will be- the park as been very busy since the 50th started. You are looking at a couple days leading up to a holiday weekend. I would expect large crowds, but it wont be packed. The hours on the calendar are not reflecting Disney preparing for HUGE crowds.