Great detective work! Those pesky little things add up and you did a great job of solving a good bit of them.
Regarding the MJ tach tap, there are 3 coil pack ground wires and a power feed to the coil packs. When the module grounds one of those wires, that coil fires. This means that in order to create that tach adapter circuit, you'd tap into those 3 wires going to the coil packs. At least that's the way I remember thinking it worked. I'm not sure whether the MJ tacho output can drive the Z tach, I don't remember the reason why I thought it wouldn't.
In any case, Clive (aka "EvilC") has Megajolt on one of his Zs but I don't think he's had a chance to work out the tach yet.