Use paypal Express Checkout for this.
The checkout wizard adds express checkout as part of paypal payments pro.
after running the wizard, you can remove the checkout form on the checkout page and only leave the express checkout button.
"The other thing I'd love to be able to do is not have to integrate the eCart system into purchasing a product."
PayPal will not accept a transaction without a product associated. The customer needs to select a product before they begin the checkout process.
The gateway needs to know the price of the transaction before they will start processing it.