Can I substitute boneless turkey breasts in a braise calling for bone in chicken thighs?

Yes you could, but I'm afraid you would sacrifice a lot of flavor.

I don't see why not, but overall cooking time will be reduced as well as flavor. I would watch the "fineness" of the breasts closely. You don't want them dried out!

Sure, you can substitute it. Be sure to sear it first on all sides.