And tsar could have as easily come from Latin through Greek as from Latin through German.

Or directly from "Rome", doncha know? There was much trading up through the Black Sea to Russia from Can'tStandYourNosePulled during the 5th - 10th centuries, and maybe later. From memory, these were the days of the Varangarians, the Norse adventurers who traded everything from furs to slaves up and down the Russian river systems. While the Eastern Empire Upper Class Geeks spoke Greek of a kind, heavily interlaced with Latin, the hoi-polloi appear to have continued to speak an increasingly debased form of vulgate Latin for quite some time after the fifth century. Traders are likely to have come from the hoi-polloi end of the social spectrum, I would have thought. "Caesar" could therefore have easily travelled north in the horse's mouth, so to speak.

Just a thought.