Question: What happened to Paolo after he was left to the paparazzi?
Answer: No one knows for sure, but he likely lost his career and faced a lot of lifelong humiliation as have previously singers who were found to be secretly lip syncing to someone else's voice.
Question: Is there any particular reason why Miranda wasn't in the movie? Did the director think she would interfere with the romance between Gordo and Lizzie, or was it for a different reason?
Answer: I think the actress that played Miranda left the show just before it was cancelled. Lizzie in the last few episodes said that Miranda went to Mexico to visit family. Most likely the actress didn't want to be in the movie either.
Question: What is the music playing when Lizzie and Paolo are riding on his motorbike?
Answer: The song is "Volare" performed by Vitamin C. It is available on the "Lizzie McGuire Movie" soundtrack. A better known version of the song was sung by Dean Martin.
Answer: In the TV show, they are 13-14 years old. Assuming this is after the TV show, and we never see Lizzie, Gordo or Miranda celebrating their 14th birthdays (we see Kate celebrating her '14th' birthday and Gordo celebrating his Bar Mitzfah a year late), Lizzie and Miranda are 13-14, Gordo is 14 and Kate is 15 (it is mentioned in an earlier episode that she was held back a year in Kindergarten.) This is only an assumption, due to the TV show.