Granting child site access (WP multisite)

I run a WP mutisite with subfolder structure (on WP instance supporting multiple child sites)

I am now trying to implement the workflow where all purchases/transactions happen on the primary site, site.com
Based on which product was purchased, users will need to gain access to site.com/product1, site.com/product2, etc. Some bundled purchases will grant access to multiple child sites. How do I accomplish this?

Is it possible to have users be registered and paid at site.com via an s2member instance configured for multisite, and then have those users automatically added to other child sites based on what they paid for?

Is it possible to check whether a user has a custom capability on the parent site, from a child site, assuming that only the parent site runs s2member?