I've traveled solo on Virgin and have boarded at 3:00 / 3:30 and haven't had any issue booking the shows I want. And all of the shows have standby lines and from my limited experience LOTS of people usually get in from the standby line.
For restaurants, as long as you are flexible just keep checking the app and all different restaurants will open up. Many times I'll make a reservation, and then cancel and rebook a new one as different restaurants open up. Razzle Dazzle seems to be available the most and has some of my favorite food. While they advertise the many vegan options - they also have steak, salmon, chicken etc. - and the Vegan Green Tea Cheesecake is the best dessert on the ship in my opinion!
And of course you can always check to see if you can get in on standby, on my last night of the 2nd cruise I regretted that I still had not tried Test Kitchen so I stopped by and they got me right in.