Yes, that would be a solution, because the "fixed" secondary attachment place will always make the wheel rub the mudguard under heavy compression. Or, even if this happens rarely and is of no great concern because it won't harm the tyre, the truth is that the mudguard will be under permanent "flexing" stress as long as the car is rolling and the suspension is working. And flexing in ways it was not designed for, therefore stressing also the secondary attachment spot.
However, I think the ideal solution would be one that had the mudguard secured in two points more apart from each other. I don't have my M3W yet but I've also wondered about this a bit and if it is at all possible to get this done using the suspension arm or even better, the rear axle as support for something, than it would work well.
Since my English is poor when discussing technicalities I hope you don't mind me using your drawing to illustrate what I mean. Here goes:
