To close the thread...

I had to put a very thin spacer (the thinnest appropriate sized washer I had in my box o' washers) between G and H in my parts photo above. The square part of the frame's boss stands proud of part H by a mm or two, and that little bit prevents those two parts from touching, preventing friction. The thin spacer solved the problem and the shifter works perfectly. I might take the Dremel to the boss and grind it down a little, or maybe not.

FWIW: The right / rear shifter is shifting a Campy 8 speed RD (modified with a longer cage for the 50/34 X 11/28 drive train) and a Shimano 8 speed cassette. Although I bought a Shiftmate #1 to adapt the Campy shifting to the Shimano cassette, I forgot to install it. On the stand, it appears to shift very nicely - perfectly to my eyes and ears. I'm going to ride the bike tomorrow and see if that holds true. Just a data point in the big Shimano/Campy world.

Thanks for help and ideas.