I would say buddha is a title/state of being, the same way christ is a title. I'm not too sure where Krishna would stand, but these three are often compared to each other.

I have also thought about Mary and Kuan Yin, and yes it's highly possible. I've read somewhere of her refusing some sort of promotion to buddhahood or something, and promising to incarnate as many times and in as many places as needed to help everyone in the world. That said, she could possibly have also been goddess brighid, mother theresa, possibly aphrodite? And many other beings throughout history.

On that note archangel michael couldve been mars/lugh, gabriel could have been hermes/athena, etc.