That looks great! Your little guy is so cute. I will try to answer to the best of my abilities, I am still a new froggy friend.

1. Hopefully this link will help you here, I am of no assistance, sorry. Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding
2. The Calcium+D3 and vitamins are the better way to go. Your frog doesn't miss out on any nutrients, and he gets everything he needs.
3. I know a few members on here breed Dubias. Maybe they would be willing to send some to you, or give you more breeding advice.

Best of luck with your new little guy! I hope I managed to help!