Nothing like eight hours of sleep to restore clarity of thinking.
Here is a reiteration of my problem which might better help identify a solution:
I removed every trace of "alien" stereo and alarm wiring and relays over the past two days, and there was quite a lot of it. Unfortunately, in the process of doing so, I apparently cut a necessary power and/or ground connection to the starter.
The whole mess of wire and relays ran from that starter kill switch attached to the three-pronged starter switch in front of the brake reservoir, ran across the engine bay to the battery's positive terminal where two or three add-on wires entered into the passenger footwell. There they connected to two relays for vacuum door lock control. These ran under the door sills to the vacuum pump where an identical set of relays connected to the pump and a ground attached to the wheel well.
The most obvious opportunities for error were 1) when I disconnected the alien wires and relays from the vacuum pump -- including the ground; 2) when I removed the same type of relays and wires from under the passenger footwell -- where they apparently were spliced to the main power and ground wires. (Whoever installed all this stuff had a passion for electrical tape -- which was used instead of proper connectors.) My original concern about disturbing something under the driver-side lower dash is still a possibility.
The most obvious plan is to restore power to the ignition switch, which should enable the starter to crank, but I am not sure how to do this -- and don't want to do it blindly. Any suggestions on how to proceed?