We set an answer to this via email already, but here it is again for anyone browsing through the forum that is interested in the answer:
1 - The product level approved URL should probably work out fine, although I've not personally tested that to be sure. I recommend trying it out with a product you have temporarily priced at .01 to see if the information is properly reported to our system. If so, you'll be fine.
2 - You would be responsible for paying your affiliates as we do not ever touch any of the funds from your sales, we can only report to you how much your affiliates have earned based on your settings. Most of our affiliates provide a PayPal email for payment, but you are free to come to any arrangement for payment that you wish.
3 - The most straightforward way to handle the price variation for your product would be to create two products with separate prices. Because you will be using 2CheckOut you will be limited to using our Buy Now buttons, which won't support any kind of variable pricing for one product. You can just set up one button for your single domain price and one button for your multi domain price.