Let's take a step back because a passport or a passport card is just as good as REAL ID, so what you mean to say is without a passport, a passport card or a real id you don't fly, and that is going to be the law pretty soon.
We have flown out of TPA three times after cruises and never had an issue with being able to make a flight at that time. There is one thing to be aware of when cruising out of Tampa. Sometimes there is fog that delays docking for hours and even a late afternoon flight might be too early if that happens.
By the time you get from MCO to Miami you may not save any money over simply flying into MIA. Have you looked at changing the airport you are flying out of?
I have always been able to get restaurant reservations for the popular restaurants any time during the week. Maybe that's only true for parties of two but we have never had an issue getting seated.
They certainly cannot use a debit card of any kind (whether a regular debit card or a prepaid one) when actually making purchases on the ship, all charges are applied to your onboard account. In the past you could not use prepaid debit cards to set up your on board account, that may have changed. If you want them each to set up an account it is best to give them cash and let them set up their accounts that way.