Well it is at least a good thing that you pretty well have run it down. It is either a lift pump involvement or the lift pump if good is really struggling to get a reliable constant amount of fuel available for it's function..

When it was coming out your overflow relief valve it was totally functional. As that flow reduced to nothing the fuel supply was dropping off.

Why I suspect crud is a lift pump does not have Much ability to be intermittent in performance. Certainly it is possible but not that likely on these cars.

The makers of lubro molly whatever it's solvent is. Did not design it for instances where really heavy cleaning was required. It to me lacks the strength and perhaps has the wrong solvent to deal with animal and vegatable fats.

There is still a fair chance in my opinion that if you clean the system out well you may be fine. If it is not then troubleshooting a cleaned out system should be easy. Do check the small primary fuel filter very carefully in the clean up as it did a number on one of our senior members at one time. Gunk inside was shutting down the flow through it periodically.

You are right of course in that there is now no indication that the injection pump has an issue. In reality they get a lot of blame quite often and in the end where not really involved. The pretty obvious clue was when the relief valve was overflowing and then stopped producing anything. That is extremely abnormal. All indications so far is that the burning of wvo in the past has not hurt anything important. Your repairs of the current issue should be cheap but labour intensive.