Instead of venting your frustration that you cancelled the previous booking, why don't you just contact the hotel directly and ask what can be done?

The website simply doesn't support the RC room for points with Diamond discount.... And a RC room is not a standard room, that award inventory seems to be sold out.

But I'm pretty sure that if you contact the hotel something can be worked out for that RC room or even that family suite. Some things are really best done directly with the hotel instead of via GP. Asking nicely is normally the first step...