Buy Button Issue - Suggestons?

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I'm working on adding the Shopify Buy Button JS to an existing site external to my Shopify store. I'd like folks who find that site to be able to checkout directly on it rather than needing to link them over to my full shopify store. 

I've managed to get the code set up so that buy buttons are displayed and the cart is accessible from other pages if they leave that page without checking out (i.e. they are still shopping). 

The bump I've hit is in the actual checkout process. On the main site, I have it as an option that the customer can open an account. So there's that option during checkout. The problem comes in that it also shows on my external site. When it's clicked on, it takes them off to the main site. This is A) confusing and B) the main site does not have their cart. I'm afraid they're going to get confused and exit. 

I've been searching for ways to fix this. I could of course, turn off the option to open an account. That's my last choice. The other options are is there someway to unify the cart between the JS code on my external site and the main store? So that if a customer does for some reason go to the other shop, they can still access their cart? Or alternatively is there some way to not show that open an account option to a customer who is checking out on the external site??

Any push in the right direction is appreciated. I have searched the boards here and on the web in general and never quite found an answer to this particular issue.