A.You can prevent non-paying users from registering on your blog by unchecking Anyone can register checkbox in Options / General screen. And then you can manually register the paying members (Users / Authors & Users screen under Add New Users section) who will then have access to site contents.
A.You can restrict commenting to logged in users only by going to Settings -> Membership section and then check Users must be registered and logged in to comment
A. Go to Settings. Under The tab Reading you will find the option For each article in a feed, show . Select Summary in that. You are done.