I had look at bike last night as mate was at the pub and he uses it mainly as a road bike - no chainstays are similar to what I can see on your pics -though they were assymetrical with drive stay being much larger. I always thought stiffness was down to the seat stays - ie that big forged? crown on them.
Anyways to stiff for me. Great colour and spec. new Commencals used to be bargains at the price not so sure they are now.
I've got a spare mech hanger hanging around if you are interested £10?
Your frame was deffo the old version. The new chainstays do help cushionning the blow but you're right, the seatstays are inherently stiff.
They have stopped selling upmarket alu HT anyway as they only do carbon, which are incredible valur for money.
I don't thin the rear hanger is compatible, but thanks for the offer.