Welcome Aboard, Andy!
I would absolutely travel aboard a Disney Cruise ship as a solo traveler! If there is a possibility that you will find a friend who would like to travel with you, I would recommend that you book your cruise with two names on the reservation. The reason being, you can always change the name on the reservation or cancel the second person. It is more difficult to add a person to ANY reservation, as the reservations are based on lifeboat capacity and you might find that all lifeboats in your stateroom's category are full, making it impossible to add a person to your stateroom.
You are correct in that if you did book a solo cruise and then decided to add a friend up to three days prior to sailing, there would be no additional charge, since the
single occupancy rate is 200% of the single cruise fare.
As a solo or even a pair traveling, you will probably be seated with others in the main dining rooms. This has actually been quite enjoyable for me on my many cruises, since I love to meet new people. Some of the people we have met on our cruise travels have become our best friends!
I hope you have a wonderful FIRST cruise aboard a Disney Cruise Line ship!
Happy Cruising, Tracy V.