6. Is the email address with which I signed up the only one that I can use as a sender?
No, not at all. You are free to use any other email address as a sender for your mailings, besides of course the one used to subscribe to turboSMTP.
(You can change the sender address directly in your email client).
Remember however that it's strongly advisable not to use accounts based on free email providers (such as Gmail or Yahoo), to avoid deliverability issues.