Sorry to hear of your troubles Hillclimber - I hate electrical problems and was relieved when my XDi installation came together without at hitch.
Here's a link to my XDi installation / wiring thread that includes a few comments and questions on things I could have installed / wired incorrectly:
http://forums.pelicanparts.com/911-engine-rebuilding-forum/812816-engine-harness-multi-pin-connector-carb-conversion-83-sc.html
A while back you posted a thread that helped me ID a hesitation problem I was having with my engine with PMO carbs - a gasket between the PMO intake manifolds and heads developed a leak (which seems to be somewhat uncommon). Greatly appreciate your efforts to document your troubleshooting and posting your findings and outcome.
Good luck - wish I could help more... Keep at it and don't let it beat you.
Gordo