The thing about diaphragms is that they have to be continuous. Just sheeting it with 12mm ply might not be enough if the joints slip or open. Our standard spec for roof diaphragms is "2 layers of 9/12mm exterior grade plywood, joints staggered in each layer and lapped between layers. tack bottom layer to supporting structure, glue top layer in place then nail through both layers into the supporting structure 3.75 diam nails at 100crs, all sheet edges to be timber backed."
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The second difficulty is that there is a spreading force in the roof that is contained by the diaphragm. This has to be resisted somewhere and it's usually through fixing the diaphragm panels to each other via the gable. If your gable is insubstantial then you may have a problem.
The third aspect is that you can't jack-in a diaphragm but it has to sag a bit to generate enough bending moment to work. If this sag is noticeable then the ridge may dip in the middle. If it does then you are stuffed because you can't just wind up a cable or tie rod to pull it in/push it up again.
Tricky things diaphragms.
