The Rules of War of the 3rd Imperium wouldn't really matter here anyway- if WW2 Earth was part of any Imperium, it would of been the 1st. So, what would the Vilani do?
And who cares what the "official" answer would be? We're already talking about something that could of only happened in an alternate universe anyway.
As far as I know, there isn't even a consensus about how the 3rd Imperium is viewed. Some paint it as benevolent, others as oppressive and corrupt. Until that can be answered, you can't really say how the Imperium would approach a situation.
And who cares what the "official" answer would be? We're already talking about something that could of only happened in an alternate universe anyway.
As far as I know, there isn't even a consensus about how the 3rd Imperium is viewed. Some paint it as benevolent, others as oppressive and corrupt. Until that can be answered, you can't really say how the Imperium would approach a situation.