If the sync has stopped running, what will happen if it is restarted?
The tasks will be synched, duplicate tasks are not created, no risk to starting the service.
Can I share Planner tasks with users outside my organization?
Yes, however these guest users will not be included in the Enterprise Resource Pool and they will not see any of the resources assigned to the tasks.
Can I link one task in Project to multiple plans in Planner?
No, Project can not determine which update would apply
Can a link one project to multiple plans in Planner
Yes, you can decide which plans task should be synchronized to. One Project can have tasks linked to multiple different plans.
Can I see when tasks have been deleted in Planner from my project?
Yes, the sync tracks deleted tasks and automatically updates the task that have been deleted in the Sync Status field.