Stuck on: Out of stock

Hey guys,

I have redirected all attempts to go to cart page, to go directly to checkout using functions.php. I have an issue though, if a customer adds a product to their cart, and that product then is bought before they get to the checkout page, they will get the default “Your product is out of stuck”-message, and get a button to go back to the cart page to remove that product. But if they click the cart-button, they will get redirected to the checkout page again, and get the same message.
I have added the possibility to remove and reduce/increase quantity in products on the checkout page, so they still have the possibility to that there.

How would you guys go about fixing this?

The page I need help with: [log in to see the link]