How does is acutally work?

Hi,
I have installed and configured the plugin to the point where I have a Sandbox App ID.

My problem now is, that I can not test the behavior of the hole thing as it seems not to work in Sandbox mode.

My main questions are:
> for simple payment option
... do I need a new account for each website as paypal allows only 1 "Notification URL"
> for advanced payment option
... do I need IPN for this
... how does it work at all. The sandbox mode does not work, so no testing possible.
... Will paypal keep multiple fundraiser separate?
... What happens when I deduct money from the paypal account while 1 or more fundraiser are active? Does it influence any of the fundraiser?
... Which adaptive payment service do I need when I want to receive the money only when the (one-time) goal is met? Is it "Preapprovals"? Then which one?
/ Subscription Payments
/ One-time Payments
/ On-demand Payments

Thanks a lot. Cheers, Alex