Once on board, you can check your app daily to see if there is any dinner time slot available at one of the three MDRs. Empty tables do come up for grab if people cancel or make changes to their dinner plan.
When I was there last year, park ranger predicts Mendenhall Glacier will no longer be visible from visitor center in 10 years or less. Everyone should go and appreciate its natural beauty while you still can.
Beautiful ships, low price, average food, ships mostly going to Caribbean only in N America. You also see a lot more passengers from S America and Europe. I would not hesitate to book MSC if the price is right.
Use your Princess app. Princess posts the daily ship activities on its app around 30 days before sailing. This should give you ample time to do your daily planning and dinner reservation based on what you see prior to your cruise.