Can I create membership accounts manually; i.e. after they’ve paid me?
Yes, you can create an account at any level via the WordPress Dashboard with s2Member, and then provide access to that account via email once you get manual payment. See: WordPress Dashboard → Users → Add New
Additionally, you can create an account at the lowest level (s2Member Level 0 / Subscriber) and provide login details to anyone, and then give them access to a higher level at any time via the WordPress Dashboard.
This is available in both the s2Member Framework and s2Member Pro. However, with s2Member Pro can help streamline the process with Free Registration Pro Forms. You can check out all of the features of s2Member Pro here.