I own a copy of the regimental history for the 15th Va. Cav. which was published by H.E. Howard years ago. Your ancestor is in there and I'll be happy to share the information with you.
The 15th was not merged with the 16th because the 16th was not part of Lee's army generally. It was merged, rather, with the 5th Va. Cav. in Oct. 1864.