Have no experience of having applied fpr B visa but can answer point three. Only the London embassy can stamp a visa in your passport. So yes you can apply for change of status within the US but would then be B2 in the passport but E2 on the computer. You have to go back to the UK to be stamped E2 if you are ever going to leave the US in the future. Basically the first time you leave the US, you have to go and get the new stamp as if you arrive back in the US with a different computer listing to visa stamp, you would not be able to get back in. And when you arrive in London for the stamp, you have to the London embassy go for interview then wait for the passports to be mailed back to you, could take a week or more. So better do it before you are too firmly ensconced in runing a business.