In a Contributor project's context:
The Planning view displays the Mirrored Milestone with all the bells and whistle (mockup to be added).
In Mirrored Milestones it's not possible to plan items directly (no drag'n drop). There is a dedicated empty state for that.
It's possible to set a capacity on the Mirrored Milestone.
It's possible to breakdown the Mirrored Milestone in sub milestones (local to the project).
Only the "Planning" view is impacted. The other tabs of the the Scrum Dashboard are not broken / touched.
When there is no backlog item nor milestone, planning v2 shows an empty state (see mock-up below). As of today it is empty without a dedicated illustration.