I think the sofa bed that sleeps two is slightly different than the one that sleeps only one. They probably needed to limit the berths due to life boat limits per location.
If that is the criteria, don’t waste a bunch of money on Wonder, take a 3 or 4 day cruise on Indy or Freedom to confirm they won’t get sick and actually enjoy cruising.
You can try to find a current 5 day cruise on any O class and look at what they cut out from the 7 day norm.