He was called the Last Adam by Paul.What do you want to show with this verse now? You contradict yourself. We both testify that he was sinless, and now he says whoever overcomes like him will sit on the throne. The "overcoming like him" is not meant in the sense you understand it, since he is sinless, we are not. If one were to understand it as you do, then no man can be saved, since no man can be sinless. So with "overcome" here clearly another sense is meant.
He cannot sin, because he is not only human, but also divine; he could never sin, because his nature does not permit it.
This implies a redo of the 1st Adam, not a God man.
This implies that the last Adam had to fix the first Adam's blunder, not God had to fix it for the first Adam.
If God Himself became the last Adam, then He blew it and had to do the job His failed creation messed up.
Kinda like a boss that poorly trained his employees and does their job for them...