Can't help with the insurance but I understand that Germany is one of the only places in the EU a foreign non-resident can register a bike. Search the forum and google for 'German Export Plates' to learn more about it - you can't re-enter Germany once you have left but the whole point is to export so they are designed for your purposes.

It might be relevant to consider how you will dispose of the bikes?

If you have willing friends / family then having them register a bike and grant you permission to use it would mean you can go anywhere and it will be easier to sell it in Germany again at the end. They may face some liability but depending on how much they love / trust you it might be ok. That would also make it easier with getting a carnet and then being able to prove that you didn't dispose of the bike overseas.

Another thing to bear in mind is route planning - when I was planning my now-aborted Caucasus trip I found it a little tricky to negotiate the closed borders without too much backtracking. As one can't travel Russia-Georgia, Turkey-Armenia, Armenia-Azerbaijan or into Azerbaijan after visiting Nagorno-Karabakh I was planning on starting in Iran and then circling anti-clockwise. If, like me you have difficulties entering Iran then you will need to go Eastern Europe-Turkey-Georgia-Azerbaijan-Georgia-Armenia-Georgia-Turkey-Home. It's a bit of a pain but luckily there are a few alternate routes so you avoid too much backtracking.

I made a very detailed itinerary with places, attractions and distances around the Caucasus that I'd be happy to share if you are in need of ideas. PM me if you want a copy but I'll definitely want to see photos of what I missed out on.