I may have made it sound to exclusive but, you get the drift. That's why I'm interested in the Caucasian mummies found in China. Two-three thousand years later, the descendents of these folks will very likely 'become' members of your Lost Tribes.
Suspect you are right, and as I recall the details they fit the "proto-Celt" timing. A lot of early European (and ???) history fits the proto-Celts.