Correct PayPal Adaptive Payments flow in crowd-funding site. I'm stuck

IPN will automatically POST data to your "listener" (which you'd need to develop) in order to automate post-payment procedures. For example, you could update a database, hit 3rd party web services, generate email receipts, etc. within IPN so that those events happen automatically any time you receive money in your PayPal account. You can also set it up to handle refunds, disputes, and other events. It's not required, but often very useful.

This is one reason IPN can be useful. IPN will be triggered whether the user makes it back to your site or not. If you're doing post-payment processing procedures within your thank you page or something like that, I'd recommend you move it into an IPN solution.

The PayKey is indeed valid for 3 hours when being used as a token. When you call Pay with an ActionType of CREATE, though, it sets up a delayed payment and the PayKey is then valid for up to 90 days.

The error information will always come back in the same format in the response. You can just log or display errors accordingly based on this standard response.

Sounds like you're pretty much on top of everything for the most part.