1. No. You can be anywhere. Even pre-COVID, there's (essentially) no in person interview in SOAP -- perhaps in big city programs for applicants from that city, but most would be phone / video. So you can certainly try to SOAP from anywhere. Will programs care if you're connecting from outside the US? Probably not. Needing a visa will make it more difficult to get a spot, no different from the main match.
2. You can SOAP in anything you want. But you only get 45 total applications -- not 45 per field. So if you try to SOAP into different fields, you'll need to decide how many applications you want to use for each. I think much depends upon how you did in application season. If you applied to EM and got zero interviews, you're very unlikely to be successful in SOAP in EM, best to focus elsewhere.
Also, please don't post the same question in two places.