Assistance needed with setting up proper wordpress site

Aloha,

I’m in desperate need of guidance. I have a website that will provide premium content to subscribers. This is the basic diagram of the website. The private access premium content will be on a subdomain.

My questions are:

Can the subdomain and domain share the same user table with usernames and passwords?

Is this the best possible setup for my intention? Is that an alternate setup?

My third-party credit card payment vendor will handle collecting the username and password and return it to the site in the .htaccess file. Given that is it possible for the domain and subdomain to use the same user table?

I want subscribers to click the log-in page, get authenticated and return to the main page of the subdomain. Is that possible? If so, how?

Thank so much. I really appreciate any assistance.