Let me start by saying that I've never used Business::PayPal, only Business::PayPay::API.

1. From reading the docs, you're supposed to store the id after you've created the button. That way you have it when you need it for the ipnvalidate. (In the synopsis, the id was retrieved from the custom parameter.)

3. ipnvalidate is the method to validate your Instant Payment Notification.

4 & 5. I don't know but it sounds like you might like Business::PayPay::API better.