Taxation for Non Resident Alien Living in USA

Hey folks, hoping someone here’s been through something similar.

I recently moved to the US on a J1 visa as a research scholar. For tax purposes, I’m considered a nonresident alien (NRA), even though I physically live here now and have a Social Security number. I’m a citizen of a non-EU country but a permanent resident of Germany (Niederlassungserlaubnis), and I lived there for about a decade before coming to the US.

Here’s where I’m stuck:

1. Both my country of Citizenship and Germany have tax treaties with the US that exempt research scholars from Federal and FICA taxes for the first two years. Which one actually applies to me — the US-Germany treaty or the one with my country of citizenship? Do I get to pick?
2. Most info I’ve found says the US-Germany treaty applies, but I de-registered my German address (Abmeldung) before moving. I still hold German permanent residency and have a written letter from the Ausländeramt confirming I can keep it, but I currently don’t have a physical address there. Does Abmeldung mean I’ve stopped being a tax resident of Germany, even though I’m still a permanent resident for immigration purposes? Does that kill my eligibility for the US-Germany treaty?
3. While I’m in the US, does my regular research income here get taxed in Germany at all, or only in the US?
4. I’d also like to open a brokerage account with a US broker, but NRAs usually need to provide a foreign address. What do I even put down there — the old German apartment I de-registered from?
5. And finally, if I do invest through that account, what are the tax implications for capital gains, and do I still have to report anything back in Germany while I’m in the US?

If you know the answer to any of these, I’d really appreciate the help. Thanks in advance!