is an iPhone bought in the US compatible with Germany?
Yes, the service coverage of Apple products is worldwide. Though having AppleCare+ will save you some money because you may only have to pay for shipment of parts, etc. Refer to section 6.2 here.
Yes, as all Apple products come with standard power adapters and they're compatible anywhere voltage-wise, you may just have to use a universal power adapter in Europe. Otherwise, you can just use the USB to Lightning cable and use your own power adapter there in Germany.
See answer above.