Peter, it seems like you covered everything I was going to suggest. However, I still think this is fixable without having to change the mirrors (or dance on them). With my baby all tucked away it's a little difficult to take photos of my mirrors now, but I don't think it applies because your mount is different. See below and send me a pic of the joint. The set screws sounds like a good idea, but they would have to go right through to hold. I still think my combination of rubber and lock washers might work.
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