Jon, I'm revitalizing this thread. Tonight I rewired my SpeedoHealer as you suggested. I do feel better about the concept that the ECU will now be getting the stock unmodified signal as it was designed to do and the corrected SpeedoHealer signal now runs just to the instrument panel and speedometer.
So, first of all, a huge thank you to Jon (Coderighter) for his excellent research on this and for sharing his great solution. The only thing I did differently is that I removed the pin from the instrument side of the light grey connector and soldered my new wire directly onto the upper part of the pin contact where the original pink wire is crimped. This way I can remove the heat shrink tubing, desolder my added wire and return the pin contact into the grey connector and restore the factory wiring.
My concern is, having now split the speed sensor signal into the ECU and the SpeedoHealer, if this signal is not designed for a high impedance input, I have just increased the load on this signal. I mean, if the speed sensor's output is a whimpy hall effect type signal that requires special receiver circuitry for amplification etc., then I have just seriously compromised the circuit and, hence, the reliability.
Now I am inclined to research the speed sensor and what type of signal it generates. The Honda FSM refers to it as the VSS (vehicle speed sensor). It is maybe a magnetic field anomaly detector or similar. It doesn't touch anything but rather is positioned very close to the front sprocket's teeth and detects them as they whir around. As such, it seems to be a rather delicate signal. Power is applied to the VSS, along wi reference ground, and it generates a modulated gentle signal of some sort.
I wonder how delicate this signal might be and whether it might be affected by the apparent load of the SpeedoHealer's corresponding signal input circuit which, like the ECU, would be designed to amplify this wonky special signal of the VSS.
Meh, that's just the anal retentive electrical engineer part of me over analyzing and worrying. Bottom line is, if it works, it works! I'm gonna have two fingers of scotch right now to calm myself down :-)