I've been told that the first account of the Buddha's given name being Siddhartha Gautama is in the "Aśvaghoṣasyabuddhacarita", and I've read that it's also in the "Buddhavamsa." I haven't read either. The Aśvaghoṣasyabuddhacarita is a tough read and I'm just not in the mood right now, and I haven't found an english translation of the Buddhavamsa.

My question is this: are these the 2 earliest known instances of the Buddha's name being provided? Also, since the Buddhavamsa is thought to have been written 500 years or so after the Buddha lived, why would Theravadans add it to the Pali Canon?