Here's a thought I just had about when I first got my jag. My neutral safety switch was broke, so I by passed it. Did not have a reverse light and it would start in gear but if your safety switch has shorted out or is damaged from say a rock hitting it from the road, then it's possible. May be a long shot or not at all. I've read a few of your post(not all) and this is what just dawned on me as something to consider.
On another note, I just recently had to deal with was the actual ignition switch(where you put the key). My started was sorta dragging and sometimes would do nothing. I'd fiddle with the key and it would work. Sometimes it would just quit running at stop lights(that was a couple different threads). I did all the starter, alternator, battery things. checked this checked that. Found all sorts of problems but still had this issue every now and then. A few months ago I was a a junk yard in another state killing time and all i could find that was usable on this 73 jag was the ignition switch. grabbed it, threw it in the bag and forgot about it till last week. Got it changed out and now it works like it's suppose to. With this in mind I have a question. when you turn your key to the start position and it's not starting. Are the dash lights on or off, the battery, temp, and other dummy lights is what I'm referring to. On mine, they would go off. When I took the old one out, it was obvious that was the culprit, but I wouldn't attempt it without another one on hand. Mine just sorta fell apart once I moved the wires and started fiddling with it.
My 2 cents.