Tracy,
I certainly would recommend you keep trying to get a reservation. If the time they are quoting is not ideal, I would take it, but keep calling back. If something releases, you can always change to a more amenable time.
That is plan 1. Plan 2 is to still walk up to the podium at the time you would
prefer to eat. Getting a table is not out the question. Sure, late May can be a bit busy, but that doesn't mean that you are going to be shut our for sure. Since this is clearly a high-point requirement for your trip, I would definitely try.
To give you some hope, I have found that often this restaurant can be a bit over booked, but under attended. Why? Well a free meal voucher at
Planet Hollywood has been a regular part of many Walt Disney World vacation packages over the years. So people naturally make a reservation to take advantage of the savings. But when they get to Orlando, often they find themselves in other parks or locations. Some call and cancel, others just don't show up. So walk-ups can often stand a decent chance of getting in.
Have a magical (and delicious) vacation!