Yep, email address is a unique key, so you can sign up from the home page with the same email address. As you've probably seen the link from the settings page actually takes you to the home page (with your email address as a query parameter which it fills in for you).
I'm not sure how I could make this clearer without cluttering the experience for the first time user. Maye store your email in a cookie and fill it in when you enter the home page?
Maybe change the link that says "go back to home" to say "add another monitor" or "manage your monitors" something similar - since that's really the only action you can do anyway.
I'm not sure how I could make this clearer without cluttering the experience for the first time user. Maye store your email in a cookie and fill it in when you enter the home page?