Hi garnettsmith, welcome home. Glad to hear your cruise went well. Sorry the entertainment wasn't that good, but it sounds like everything else went really well. You are correct that Carnival should tell their customers up front that pre-paid gratuities cannot be adjusted on board. However, I don't know of anyone on board that doesn't deserve their portion. They all pull multiple jobs on board. Sometimes they're even the entertainment. But you're correct. If you don't know up front, then you'd have a dickens of a time on board. No one wants to go on vacation for a hassle.
Thankfully in the 20+ cruises I've been on, I've never had to worry about the crew not deserving their gratuity. Sure hope in the future, that holds true, but cannot foresee why it wouldn't.
I, too, think you're a brave soul with that many women. Can't wait to see why you select for your next adventure.