chicago ~ have you tried contacting a TA to see if they have any available rooms? They also might be able to tell you what company might be holding rooms and when they would be released...
flow ~ my BIL stayed in the old tower with his 1 year old. They booked through our TA because the rates were waaaay cheaper than Dreams or any of the travelocity, expedia, etc. sites. You might have you SILs check with a TA to see what they say about the babies.
As for Save the Dates and Invitations, do what makes sense for you and your group. We only sent about 70 Save the Dates, but 170 Invitations becuase the invites included the AHR. We had 50 of our best friends and family with no drama at the DW. That's just what worked for us...
april ~ personally I wouldn't worry. We had one guy who didn't book a room and just added on to a single when he got there. I started to freak over it, but then took a step back and realized that he is a grown up and should be able to handle a vacation. If you are worried, you might try hitting up the concierge to check on the bookings, but there is a chance that the TA hasn't sent in the names yet.