Sorry to revive this resolved thread, but it just occurred to me that the proof of our interpretation was in the RB all along:
A9.21 ...Remove the CA counter at the end of the current phase [EXC: a SW MMG/HMG with a fixed CA, and whose operator was pinned in the PFPh/MPh, retains that fixed CA until the end of the DFPh].
If a pinned unit couldn't again fire a MG that maintained ROF previously, why would this EXCeption be needed? (the same EXCeption is made for GUN CAs under C5.11, last sentence).
...first I thought "Doh!"...then I just laughed. ?
...sigh.