I agree with @ben on number 3, assuming the list imported is cleaned prior (as he suggested). What we don't want to find is the OpenTechSchool Mailchimp account banned because a lot of people clicked on the Unsubscribe link, on the very first newsletter!
Also, are the only email addresses we are importing OpenTechSchool.org email addresses? Am I right in saying that only organisers have OpenTechSchool.org email addresses? What about the OTS attendees and coaches from each chapter? I think we discussed this during the 1st Conf call - that the newsletter is for everyone involved in OTS (organiser, coach or attendee).
For London, I think we have some email addresses for attendees and coaches (but only because some venues required them). We can do the same exercise of telling them that we will be adding everyone to the newsletter mailing list, and they should let us know if they want to be removed prior to the list being imported.