I can understand your being upset, when you have something pictured in your mind and something throws a monkey wrench into it, that can hurt. BUT we have cruised on
DCL three different times, and the first time we didn't even know about booking things ahead of time (talk about being new at things) when we got onboard the Magic, we were directed to one of the nightclubs where they were holding dining changes and the Palo bookings. We were able to book onboard, on our 2nd cruise we ended up dining in Palo 2 times because we waitlisted, and on our third we would have dined 2x again but missed the phone call because we left early to go shopping before dinner.
So don't give up hope, when you get to the terminal, you'll get a personal navigator, read it, find out where they are holding the dining changes/palo/remy bookings and go there first thing. See if they can book something, talk to the dining manager, and then if nothing can be done waitlist and you most likely will get called over 7 nights people will decide not to dine at Palo and cancel.
If for some reason that there is no way to get in to Palo, take a deep breath it will be ok, sometimes things happen for a reason, and you'll find a perfect spot for the moment. Some other ideas are to go down to the 4th deck and take a walk around the promenade deck at the back of the boat as you watch the boat sail and the sun is setting make that moment the special moment, take a bike ride on
Castaway Cay and when you feel the moment is right ask her there, try and talk to the cruise director and ask him to work with you on doing it onboard in a special way and see if he/she has a suggestion.
But the best piece of advice I can give you is to take a few days, calm down, take a deep breath and go into this cruise instead of focusing on the feelings of negativity and find things to be excited about, and to make the most of the cruise, wherever you ask her, she is going to answer because she loves you not because you did or didn't do it in a restaurant, and its going to be the start of the next chapter of your life, try and make it positive rather then negative by focusing on something like Palo.
Sending you some good thoughts.