Hi Shamsham,
1- I use "OpenPhone", it's a service you pay for online through an app. You can an American number, unlimited calls to the US + unlimited messaging. Works only when you're connected to the internet. It's not exactly cheap, but works well for me.
2- Depends where you live in the US. All information here:
https://www.uscis.gov/forms/filing-guidance
3- No idea, I applied one month ago and still did not get the notice for the appointment. I think the delays are very lengthy these days because of the Covid backlog.
4- The guidelines state that you should stay in the US until you submit your biometrics. But many people leave for different reasons specially if the delays are too long until the biometrics appointment. So far, USCIS seems to be tolerating it. Your call.
Hope that helps,
K