Can I disempower my Ambessa, Matriarch of War with Profiteer to empower my Kayle? Do I control my own legend?

Yes, you can disempower Ambessa, Matriarch of War with Profiteer. And yes, you control your own legend.

We don’t typically consider the possibility that any player controls a legend, because control of legends cannot change, but you do indeed control your own legend.