Setting up a separate shopify store to sell in US

[ProtectorUS]

Hi all,

I currently have a shopify store selling to customer in Hong Kong, and I am looking to starting selling in the US. 

The idea is to set up a separate shopify store for US customer so I have control over inventory, shipping, currency and items shown to customer based on their location. 

Below are questions I have while trying to set up a new shopify store.

1. Can I set up the subdomain of my Hong Kong store as the primary domain for my US store? I get an error message when attempting to do so. ("A store with that domain name already exists")

2. How do I create a button for customers to have the option to enter both sites (separate shopify stores)? I have used a geolocation app that redirect based on IP but customer do not have the option to visit the other store.

3. I want only the main shopify store to show on google search. Can it be achieved by implementing hreflang?

I would appreciate any help with my questions above.

Thank you and good luck!