I have to say the use case you’re describing is incredibly elaborate and specific use case for Ghost Members. The complexity of this leads me to believe that it wouldn’t be a good idea to house the project all under one platform, regardless whether the platform is Ghost or not.
From what I’m getting in your post is that they are doing a great job of managing pricing tiers in Patreon, and Ghost can be used as a tool for providing content to those Patreon subscribers.
If you take a look at the link I sent it shows how the handlebars templating within Ghost can pull out key subscription information from a Stripe account. If they are managing their Patreon subscribers through Stripe then the Ghost site could read that subscription information and lock content depending on the Patreon subscription type. Signups would still continue to happen on Patreon, but people wanting to see Ghost content that they paid for could use their same email to sign into the Member site.
I hope that makes sense