RBAC currently doesn’t handle relations (yet) but we do have a request for that: Author can select relational content that another user created. · Issue #7696 · strapi/strapi · GitHub
But regarding your second point, we handle the list population based on what we call conditionals (WIP PR for that documentation: https://github.com/strapi/documentation/pull/187 )
However custom conditionals and modification of the default existing conditionals is only an option in Enterprise licenses, with the cheapest option being our Bronze License.