Same thing happened to my friends dad,who had a '66 Mustang 2+2 he bought new--he read about that trick in a hot rod magazine ,"how to make a cheap anti-theft device:"...
We went to a cruise night in it once and we wanted to walk around and look at the other cars & trucks,so he opened the hood and put in the fake coil wire,made of 1/4" vacuum hose..
After we walked thru the place and hung around till it was getting dark,we got in the car,he started it up,and drove away...he noticed it wasn't running "perfect"--then I pointed to the coil wire sitting on the floor next to my feet !
He pulled over and said WTF?..how the hell can it run ?...and as you said,the carbon black in the hose was a good enough conductor to let the spark still fire the plugs..not as good as the real coil wire,but good enough to foil its use as a theft deterrent..
He later made a fake one out of clear vinyl tubing painted black..that one did work,but the engine would still "almost" start!..
As for a control module,I've only had HEI ones fail completely ,not "halfway"....usually they either work or don't..but I suppose one could develop an intermittent failure condition..pick up coils do that more often though..
