I've noticed that my right side Brembo has a shim to reduce the mounting hole diameter. That suggests to me it's borrowed from another application.
As for the master cylinder - it's possible your single caliper model will work just fine with duals. I've used 13s and 14s for duals and they work just fine. The 12mm may be fine also, the problem being excessive lever travel. Doesn't hurt to try it out, as long as the brake clamps before squishing your fingers. :-)