You can checkout using a credit card from Paypal, or using a credit card all from the beginning... which is better... (Stripe, Bluesnap, braintree...etc)
This plugin, uses the same concept Clickfunnels is using already since the problem is getting your sales "tracked" within Clickfunnels (One time payments & subscriptions) other than that, I could easily make it work like Shopify does! but really what's the point! You'll end up losing all the CF magic.
That is IMPOSSIBLE, It's against Paypal's TOS! They have to confirm the purchase for all upsells/downsells within a secure Paypal page... the good thing, is they have to log in only once, and all other subsequent times should only confirm by clicking the "Buy Now" button on a secure Paypal page, and they'll be brought back to your funnel.
The point of having this plugin is not replacing your credit card processor, but instead boosting your conversion rates (Millennials, European countries, and South America...) all these prefer using Paypal, and by simply using the plugin, you can notice a big increase in your conversion rates and a huge decrease in failing payments through Credit Card.
hope that helps!