I'm surprised that you couldn't find an auto electrician more familiar with common Ford
ECU problems.
I've been in the auto industry for 40+ years & these days do a lot of work in the EFI area.
Most cars' ECUs are very reliable, but every now & then we see regular failures on one particular model. Arguably the common failure with Ford ECUs (of all series) is the idle circuit in EF 6-cyls. This was definitely your problem all along.
With the idle circuit not working, you will get intermittent hard cold start, occasional stalling, worse in high load situations, i.e. A/C on, in drive, headlights on etc, etc. At one time, a few years ago we used to sell several of those units each week.
Dr Terry