EDIT : There is a pic in my ST1300 Service Manual that shows the ground strap is connected at the end of the starter.
Also, it helps when diagnosing an intermittent problems such as yours to wiggle the wires while you are checking for continuity like when you did the continuity check from the knock sensor connector to the ECM connector, to try to induce a bad connection, if it may exist. You may need a helper to hold the meter leads while you wiggle the wire going into the knock sensor connector and into the ECM connector. While wiggling the wires, the meter should display a nice steady reading near zero Ohms. Should read about the same as when you touch both meter leads together. In fact, you should read just about the same reading when you Ohm out between the negative battery terminal wire and the engine block.
When you switched the knock sensors, was there much corrosion ( white stuff ) on the threads of the knock sensors ?
Jim V. ( old electronics tech )