The Armenian word for seven is "yotta", which sounds suspiciously similar to the Greek prefix for a factor of one septillion (though AIUI it comes from the ancient Greek word for eight, not seven). Is there evidence that these words are related, or is it just a coincidence?
It is a coincidence.
The prefix "yotta-" is in no way Greek. It is a modern, invented, coinage, derived from Greek ὀκτώ (okto) by a very roundabout and idiosyncratic route.
Armenian յոթ (yot') derives from Old Armenian եօթն (eot'n), older եւթն (ewtʿn), ultimately from IndoEuropean *septḿ̥