IMO, it would be a lot of food. The entrees at Beaches & Cream are lighter fare (sandwiches, etc) so we have more room for the giant sundaes after we eat there. You'd also need an ADR to dine-in at Beaches & Cream, which means you'd have to have two somewhat back-to-back ADRs with the right timing. Maybe it would work if you spaced it out enough so you aren't rushing at dinner and have time to digest a bit. You could even share a sundae. An alternative option is to get a sundae at the walk-up to-go window at your own convenience, but they don't offer the specialty sundaes there. You could also try your luck inside at the counter which is first come first served. If seating is available, you can get a specialty sundae at the counter.
We really like the food at Beaches & Cream and go there for lunch often, so I would eat dinner there personally. But as I mentioned, its lighter fare and super casual so very different than the kind of dinner you'd have at Yachtsman.
Not that I know of. Perhaps at the to-go window which is walk-up? I can't think of anything on the dine-in menu that would allow a snack credit.