OK, send a message through skype is you still need help.
For the issue you are reporting where some users are getting a failure on directing back from paypal, this is probably a case of domain switching.
when items are added to the cart, they are stored in a session.
If the user browses to your site using the domain name without www in front and starts adding items:
and is then redirected to the pp_confirm page with www in front of the address:
this is considered 2 separate domains and 2 separate sessions. to prevent this, you can use htaccess to force the address to always point to www: