You will have to do your immigrant visa processing in your home country. He files the visa petitions in the US at the service center and then you will have to wait to go through the consular processing in your country.
You will not be allowed to immediately enter the US, and cannot enter on a tourist visa as you are an intending immigrant.
The process for you to join him here will take about a year.
No, it doesn't matter if you marry in BVI or your country in SA. Your spouse will still have to go through the process of petitioning you through the service center in the US. When it comes time for your immigrant interview, depending on your status in BVI, your interview will be scheduled there or in your home country.
Yes, the new stamp supersedes the visa stamp which is just to allow her to enter to process for the admission as a resident.
She will get a 10 year card.
Yes, she can leave and re-enter on the temporary stamp while the card is being produced.
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