Can I capture email with ConvertBox [and or] Fluent Forms and send the email subscribers to my WP sub-domain dedicated to MailPoet?

I like the idea of MailPoet, but I'll need to run it on a dedicated mail.xxxx.com WP sub-domain as I don't want to add further code/plugins to the primary domain. Will MailPoet work on sub-domain?

Does MailPoet have a double opt-in feature to better ensure subscribers actually want to receive newsletters?

Last question, several sumolings have been talking about agency level use. Thus, how many codes need to be purchased to access agency level? And how does a new client set up and access their MailPoet sub-account as my client?

Thank you for taking time to answer my questions, and I look forward to your feedback.