1. Yes, in fact you can create multiple different paypal buttons (and payment amounts) for each level. S2member just ensures that the payment has gone through, no matter the price in the button, and gives access to the user.
2. You can add anyone at any level for free through the normal “add user” option in wordpress. Just select the proper s2member level as their “role” when creating them. You can also add an EOT (end of term) date to their profile when creating or updating the user’s info if you only want to give them access for a limited amount of time.
3. Currently S2member doesn’t have this option. However, I have read of other owners using other third party plugins to manage this part. I’m not sure what they are so hopefully someone else will chime in on that.
4. The paypal button generators will give you the option to allow a free trial period when generating your buttons.
My suggestion is to go through the video tutorials, especially the one below so you get a good feel of how it all works before jumping in.
http://www.s2member.com/videos/
Hope that helps.
Dave