I am an old graduate, currently working in Japan and following the USMLE journey

I am an old graduate, currently working in Japan and following the USMLE journey. I have completed step 1 (average score) and I am going to take step 2 in August or September. After that, I plan to quit my job in Japan and go to the US for a research position and take the CS+step 3. I am a pathologist so for sure I will apply for Pathology. Probably I have not much hope to apply and match this year (my CS result will not be ready by mid-September). I have 2 questions regarding US VISA that need your consultation:

  1. I got an acceptance from a professor in a large cancer center in the US for an unpaid research position. However, the issue is the US institution might not be able to support a visa J1 for me. My question is whether visa B1 is eligible for a research job in the US? I checked on the internet by myself and found that visa B1 might be not eligible for such works. I just want to have your confirmation about this.

  2. The second question is about visa J1 which have a requirement of two-year home residency after completing my research in the US. Will it affect my chances to get a visa H1B in the future if I match to US residency?
    TYVM