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.

No, Project can not determine which update would apply

Yes, you can decide which plans task should be synchronized to. One Project can have tasks linked to multiple different plans.

Yes, the sync tracks deleted tasks and automatically updates the task that have been deleted in the Sync Status field.

Yes. This is very popular request from those currently managing a project in Planner and need to see the visibility and status in Microsoft Project