A Non B stands for “business” visa. It’s for employment. It’s where a company employs you (and pays you) to work for them.
A Non O (volunteer) visa is for people that “work” at an institution but don’t get paid. A work permit is still a requirement to do that though regardless that you’re not getting paid.
It’s actually probably the best suggestion. Contrary to popular belief most offices will try to help you rectify an error if they can. It may not be an issue for them anyway.
That’s probably going to be at the discretion of your immigration office. As a guess I’d say they may well reject your application and you’d need to start over.