Welcome home, congratulations and hurray for you!
Some answers to your questions:
1. Availability is always an issue when you're booking
DVC vacations...you can book starting 11 months from the date of your check-out at SSR (your home resort), and 7 months from the date of your check-out at any other DVC resort. I don't know about the non-DVC booking options, as I've never used them. WE've had little trouble getting our desired reservations, altho sometimes we have to use the wait list. But I do know the sooner you book, the better your chances of getting the resort and villa of your choice.
2. No, DVC does not provide rollaway cots, sleeper chairs or any other kind of extra sleeping space. At SSR, a studio has a queen sized bed, and a pull-out sleeper sofa (I think a double, but possibly a queen). No cots or sleeper chairs possible. The one bedroom also has the pull-out sleeper sofa in the living room, and a king sized bed in the master bedroom. The two beds in the bedroom option referred to by a poster above is a two-bedroom villa set-up, not a one-bedroom. You CAN bring an air mattress (or sleeping bag)...there is enough floor space to accommodate that...but Disney won't provide it. As I recall, the bed (in both the studio & the one-bedroom) has multiple pillows on it, and there is bedding for the sleeper sofa...but you're on your own for the bedding as well as the "mattress" (sleeping bag or air mattress) if your kids don't want to share the sleeper sofa bed (or the regular bed)...
3. Annual passes are good for 365 days from the date of their first use. Date you purchase doesn't matter. For example, ours our good from 11/22-11/21 each year--and I purchased our renewals in June. Didn't change the "effective date" at all.
Hope this helps, and congrats again...