Answer
Neither is possible, per IFS Support: there's no way to have the navigation tree show specific sub-links instead of the Lobby Overview link when a Lobby is set as the homepage, and there's no way to bulk-default a group of users' homepage to a lobby — it has to be configured individually per user. The practical workaround for the second point is to copy the homepage/lobby link value into each user's own profile setting (a one-time per-user setup) rather than trying to apply it at a group/role level.