It's also rumored that Leonard Nimoy filled in for Welles on some of the voice work, possibly after his death.
The comics are probably the closest thing to canon for Unicron's origin. Cybertron is actually the body of Primus, who sealed himself and Unicron, his opposite, within two planets. Billions of years later, they both discover how to reshape those forms. Primus creates the Transformers as an army to finish the fight against Unicron, but the civil war mucks that up. The Matrix is actually a piece of Primus' essence, which, when wielded by a good Transformer, can destroy Unicron.
As for Beast Wars, there's no connection. It's a separate series, with its own continuity. Optimus Prime and Optimus Primal are not the same character, and are not related in any way.