Interesting way of putting it, and the more I look at the rules relevant the more I think the blame is completely equal.If the driver in front is blocking the overtaking car then I agree that a penalty should be give without a doubt. If a car is out of control, it is the drivers responsibility to get the car back under control which was being done.
In a normal case, the driver doing the overtaking, it is their responsibility to make sure the move is done in a safe manner but on a tight circuit like Canada when the driver behind saw the car go off the track, did he not realize that there is a "60/40% - Toto" chance of coming back onto the track and 60% chance of going across the track?
By not allowing the car in front to rejoin safely by putting his car where he put it , as an overtaking car, did he not create the dangerous situation? (Renault fan by the way - this post is no hate towards Merc)