I emailed Paradise Beach directly, as I had the same questions as the OP. We have been there 4 previous times and looked forward to going there on our next cruise. We do not eat a lot, and I am the only person who will drink a few beers, so the all-inclusive option doesn't make sense for our family.
The email response I received indicated that, yes, we could still enter via the $5 entrance fee with the $10 minimum spending on food/drink per person. The caveat is that they are at 50% capacity at this time, and don't take reservations for this option. With at least 4 other ships in Cozumel, I would expect them to be sold out, and we'd be stuck trying to find another option if we take our chances and just show up without reserving the all-inclusive option.