Unable to display a custom page within an index page

Site URL: https://red.org

Hi, I have a custom page (built in developer mode) here: https://www.red.org/team-gallery and am trying to add it into an existing index page (built via the Squarespace admin) here: https://www.red.org/meet-our-team (page password: MeetThe(RED)Team) but it's not displaying.

Is there no way to have a custom page display as any other page would within na index page?

If not, is the reverse possible? To have a custom index page (created via dev mode) contain pages that were created in the Squarespace admin (not created via dev mode)?

Thanks,

Chris