All of the nights must become available at once. The system will not "hold" or "reserve" nights waiting for the rest to become available.
MS recently changed the waitlist processes. You can no longer waitlist for each day on an individual basis. You must waitlist for the whole stay.
What you can do is call and book individual days if they are available. Then you can modify your waitlist to match what you actually need. No guarantees that you will be able to get everything you need.
Shorter length waitlists do have a better chance of coming through (IMHO).