ERAS questions

1. If you can get another letter from someone who knows you well to use as your 4th letter, that would be preferred. Otherwise, I think it's up to you whether you use the chair's letter at places where it is not required. I don't think there's a big difference between someone with 3 strong letters and someone with 3 strong letters plus the generic chairman's letter (they're almost always generic and relatively neutral and those of us who read applications know this and don't hold it against you). We had many applicants with 3 letters last year, and that didn't hurt them at all - that's better than reaching for a 4th iffy LOR.
2. Not necessarily - if you apply broadly and choose programs that you are competitive for and who have a history of taking IMGs, you have a good chance at getting 5+ invites. You just can't afford to be picky about where you apply.
3. Depends. If there's a program that you really like and think you'd be a good fit for, but you don't get an invite within a month or so of applying (maybe because they filtered out your Step 1 score or IMG status), a former supervisor could be helpful in advocating for you getting an interview. Otherwise, closer to the time when programs rank applicants (January) it could be helpful, but not all programs will put weight into a call from a mentor or supervisor, so don't necessarily count on this helping.
4. I wouldn't draw more attention to your Step 1. Your Step 2 is fine, so let the programs see that you improved without going into further details. If you had a good family/personal excuse for Step 1, that would be a different story, but you don't want to tell programs that you are a poor test taker, because they'll see you as a potential liability towards their boards pass rates.
5. Unfortunately, I don't think so - I think you're stuck with the ERAS version.

Good luck!