Trying to get some quick answers in to all your questions...

Checkout Complete happens when someone makes it through the checkout review form's order submission. It doesn't necessarily mean the order is paid. For example, they might be sending in a check... or in the case of PayPal, we may not have received confirmation that their payment went through (or they could be paying with a pending eCheck).

By default, you can't use multiple e-mail accounts with PayPal. However, if you're simply using PayPal Website Payments Standard, you can use hook_form_alter() in a custom module to adjust what e-mail address it sends in based on your conditions. This would be great for something like one account for micropayments and another for regular orders.

Lastly, the Sandbox is just difficult and annoying to get into. You won't actually be charged for things when setting up your account, they just make you enter in all the data like a normal user. They weren't smart enough to provide basic dummy CC and SSN data last I signed up, though. Check the bottom of the WPS docs for a link on setting up a Sandbox account.