Welcome to the community forum. Great to hear that you’re considering using Frontity for a project.
Frontity cannot support the admin pages. There is no ‘headless’ provision for the admin pages as admin page content is not accessible via the REST-API. Content needs to be managed via the standard WordPress admin pages. That content is then rendered by your Frontity theme instead of a normal WordPress theme, with the benefit of all the advantages that Frontity offers over a standard theme.
Frontity does have support for multisite. Please see: https://docs.frontity.org/learning-frontity/settings#multiple-sites.
Hope this helps.