In general you can create the warehouse view prior to any plant view if you distribute the creation of material masters(theoretically you could even begin with the WM view), so an update would not even technically be possible in that case.
If the user enters the plant then he enables at least SAP to validate the correctness and existence and subsequently SAP can update the status.
In data migrations I always decide myself which status I submit.
I have not yet tested the manual case, but I trust you, and I see no chance how an update of MARC could be forced. Have you seen OSS note 2145027 , not quite easy to understand but certainly valuable information on the status updates. I found it last week and distributed it to my colleagues from the migration team.