I'm seeing some storefront mutation apis which are not in document, especially checkout related. Also the guide is not mentioning them, I'm not sure how to use them, I hope the API team can help fill the gap.
For example, those several CheckoutCompleteXXX apis.
And CheckoutCustomerAssociateV2, I don't know how to use it. I tested, and was hoping to see user logged in in the checkout page after associated, but it wasn't.
For checkoutCustomerAssociateV2 you first need to customerAccessTokenCreate in order to get a token.
The checkoutCompleteXXX mutations are for the Android / iOS / Unity SDKs which also use Storefront API. However, those aren't intended for regular web clients.
Hope this helps!