Right, for OpenID Connect, the setting is called openid connect overrides email, not auth overrides email. When it’s enabled, every time a user logs into Discourse through OpenID Connect, their Discourse email will be set to the value of the email address they use on the OpenID Connect auth provider site. That means that if a user has changed their email address on Discourse, it will automatically get reset to the value of the email address they use on Memberstack.
You can actually disable the email editable site setting to prevent any issues with emails getting out of sync with the auth provider. So if you disable email editable and enable openid connect overrides email, users will only be able to change their email address by changing it on Memberstack, then logging into Discourse via Memberstack.
I don’t think so. After someone clicks the “Sign Up” button on Discourse, Discourse will redirect them to the authorization_endpoint that’s provided by Memberstack. Possibly Memberstack allows this to be configurable, but you’d need to contact them to find out.
Another possible approach is that it might be possible to add a link to the page custom page to the Discourse header. Maybe with this theme component: Custom Header Links. Ideally you’d be able to hide the link from logged in users with some CSS.