Apparently you're from Berlin, (I'm sure this is google-able)--are foreign (US in my case) students able to enroll in classes without fees, difficult to obtain visas, etc? Are many courses offered in English?

As far as I know there are no additional fees for foreign students. A lot of Master courses get offered in English. I think it should be easy for US citizens to get a visa.

But I'm a German citzens so I don't know the details from the perspective of being an US citizen well.

I'm not really sure what I want to do with my "understanding of the material,"

How about spending a gap year to think about what you want to do with your life before starting a new degree at university?

Unsurprisingly, I don't have the raw intelligence of Yudkowsky, so I have doubts about how well someone with my skill set will be able to make progress without support/credentials.

I don't think that raw intelligence is the most important thing. The important thing is to be willing to do work in a way without a clear path.

Having social skills is also important. If you have marketable skills and network well, credentials aren't important.

If you are truly interested in biology and science I would suggest that you do quantified self style self experiments. Start a blog about them.