I am talking to people on the phone not doing it online
If it's the reservation CMs that are saying that there's no more availability within your desired category, I'd believe them.
It may be possible that someone may cancel at a future time, and you'd just have to keep an eye on the availability to catch one, if so.
I think the best choice at this time would be to go with lanejudy's suggestion of booking that last reservation in a different category (in your desired location) and switching it, if possible, later.
EDIT: I see you said "The reservation agent agrees the cabins are available."
If there are rooms available within the category, then they should be able to book them. It makes no sense that, if you you have, say, 6 rooms you want to book, and there are 10 rooms available, that you can't book 6 of them.
If indeed it's a "lifeboat issue" it's entirely possible that you can only book 5 rooms in one area, but the 6 room would have to be in a different area. I don't understand why that 6th room couldn't be booked in a different area. Unless it also is at lifeboat capacity.