I guess it would still be possible to use the original MC with electric brake booster. However, on LHD cars it's one complete unit, including the ABS valve block. I have not investigated whether it's possible to separate the two, but I'm not such a big fan of the standard brake booster. The accumulator is very old by now and loses its charge quite quickly, meaning the electric pump is running every 2 to 3 times you press the brake pedal. Next to that the unit is very heavy, so I think a normal servo would be much better.

Something I also like about using the Granada MC is that the Granada uses the same size brake pistons front and rear as the Cosworth, so the front/rear brake balance should be no problem either.

I think I have found a way to make the magnetic ABS rings and sensors work on the front knuckles. So that means I should be able to use the MK60e5 system, so I can start gathering more parts.

BTW, you said the standard ABS sensors are 23 mm wide, but I measured the hole in the rear arm and that is 18 mm. Did you maybe make mistake in converting from inches to mm? Are the E46 sensors then also 18 mm wide? And do you perhaps have a picture of the E46 sensors? It's still unclear to me which sensors I can best use on the rear. Maybe the E46 sensors, but if the hole for the bolt doesn't line up with the hole on the arm it might be easier to get a sensor from an E9x 4 cylinder car, as I believe that is rotated 90 degrees, so I could make a small adapter piece to fit in between the arm and the sensor.