Always enter and exit the US with your US passport. Not doing so is illegal.
For anything else, use your German passport, especially in the EU. That's what I have been doing for years.
I don't know if it is an issue for immigration, but you absolutely are required to file taxes every year and can get in trouble if you don't. There's a "streamlined procedure" that you can do if you've never filed taxes to start filing without incurring any fines or penalties.
Well, the definitions aren't really all that helpful. 12pm is neither before nor after midday, while 12am is exactly 12 hours before and after midday.