I agree that Celia is likely St. Cecilia, the patron saint of musicians. Here, she serves as the song writer's muse. Making love to your muse would mean that Simon is being inspired to write songs. The "someone else" who has taken his place could be another musician who is having success while Simon is having writer's block. Jubilation comes when he is again inspired to write more music. I did not know that Simon had a dog named Cecilia, but I would suggest that he chose that name because of Saint Cecilia's role as the patron saint of musicians. Maybe he hoped that, by naming his dog Cecilia, she could serve as a muse for his songwriting!