"From other threads I learned I can't get a US-Vessel-Registration"
No, more precisely there is no US-vessel-Registration. We have federal
Documentation for pleasure craft. The term registration is reserved for state registration of boats and motor vehicles.
As a non-citizen, that's right, there is no way you can be given the protection of a US flag for a boat. You must seek that status from your own sovereign, i.e. Germany.
If you will be keeping the boat in Michigan, you can and probably must register the boat as a Michigan motor vehicle. If you plan to cross over to Canadian waters, you'll have to ask the Canadians about that, and the US about re-entry. It sounds like you are a guest not a resident alien, that will make re-entry harder.
If you need more than a state registration, you will have to document the vessel with another flag--and then it will be subject to terms limiting the length of time it can stay in the US, so you may very well have to get it out of the US (and Canada) by the end of the first year. It will be governed by the terms for any visiting pleasure craft, you can find them on
http://www.cbp.gov/ where the terms and regulations for aliens with vessels are all referenced.
For now, your best bet is simply to stay within US waters and register the boat in Michigan. Then flag it in Germany before you take it out of the US for good.