I did this as well (BS in biology and went to Germany for med school), graduated on time (6 years) and will be starting residency in July at my #1 choice. It was not an easy path but was well worth it. I am not German but learned it after spending a year there in college. I will start residency debt-free and was able to set up a lot of US clinical rotations that counted for the Famulatur and PJ. The training in Germany is excellent, but very difficult. At my school, the competition level was intense. When I started, there were two Americans in my year; both didn't make it. Another one a year above me did not, either. One had a mental breakdown, one just disappeared, and one failed 3x and was ex-matriculated. However, all of were American and not half-German; they all spoke German but I think the language and cultural barrier has a lot to do with the difficulty of medical school for foreign students. If you are a native speaker, that will be a big advantage.
There is no support network at German universities either in terms of counseling and advising; you are pretty much on your own. No one will try to help you if they notice a pattern of failing or problems in courses. If you fail 3x, they will kick you out so fast you don't know what hit you (this happened to the third guy I mentioned).
I was given credit for a lot of courses: chemistry, physics, biology, biochemisty, and genetics. The integrated curriculum made it more difficult to save time, though. I did complete the Vorklinik in 3 semesters, but then I would have had less than 5 weeks to study for the Physikum, so I waited until the next semester.
Depending on your school and LPA, you can either submit your US transcripts directly to the LPA to get course credit (this takes almost a year), or contact the professors for those subjects directly, set up a meeting, show your transcript, and ask them to give you credit. If they agree, they will write you an Aequivalenzbescheinigung for the course and you can submit these all at once to the LPA. Getting credit that way is a lot quicker. This applies to the LPA in Stuttgart so depending on your LPA, the rules might be different.
Hope this helps answer your questions. Let me know if you have any more. Which schools are you applying to? If I recall correctly, applications for foreign students are due this year in August and you receive your acceptance in September. Admission is highly competitive; at my school, less than 4% are accepted, so I would apply widely.