My advice would be to do as follows
The Subscription Signup/Registration Process
1. Internet Users will go to your Membership Options Page ( which you’ll need to configure on the s2Member General Options panel ). On this Membership Options Page, that YOU will create in WordPress®, you’ll insert the PayPal® Subscription Buttons that were generated for you by s2Member.
2. An Internet User will click on a PayPal® Subscription Button from your Membership Options Page. They will be transferred over to PayPal® in order to agree to your Membership terms and pricing. You can customize the Checkout Page Style, Pricing, Payment Periods, and more – whenever you generate your PayPal® Buttons through s2Member.
3. Once a User has completed the Subscription Signup Process at PayPal®, they’ll be returned to your site, where they’ll be activated by s2Member instantly, and given the opportunity to register a Username & Password for their Membership. ( Note: they’ll be allowed to register a Username & Password, even if you’ve set ‘Anyone Can Register’ to `Off` in your General WordPress® options; because s2Member identifies the User as having paid for Membership access through PayPal® ).
s2Member will also send the User an email with instructions on how to register their Username & Password, just in case they missed the instructions after checkout. That email will be sent to their PayPal® email address. Much of this is handled through the PayPal® IPN service behind-the-scene, where PayPal® and s2Member communicate with each other.
4. Once a User has completed checkout and registered a Username & Password, they’ll be able to log in. The first page they’ll see after logging in, will be your Login Welcome Page ( which you’ll need to configure on the s2Member General Options panel ). Your Login Welcome Page can contain whatever you like. You’ll need to design this Page in WordPress®, and be creative!
It looks like you already have everything setup so all you need to do is put the paypal buttons on your membership page instead of directing users to the pro form on your monthly-payment.
I know its not the solution you want but without the ssl certificate you either have to get customers to register before or after payment and I guess in your case its better afterwards. When you have some money coming in you can always purchase a certificate and go down the pro forms route.