Jack, you have a lot of questions, and I will try.

I think your final payment is due 120 days before the cruise to avoid penalties.

Yes, waitlists clear at that time, but those who have requested lower (cheaper but same) cabins, who knows?

You will only get an upgrade if you book a "guarantee" cabin, and usually it will only take you a deck or two up, not to a prime cabin, although this is possible when a cruise is sparsely booked. I have heard of it happening maybe three times now (standard to penthouse)

Unfortunately, we know of no site on the internet whre you can monitor cabins available. There used to be one years ago, but unfortunately it dissappeared or stopped offering the service for Radisson.