You raise some very valid points there Bob, however some of the issues you have identified have being overcome, and others are by no means insurmountable, for instance:
1. you can retain the full compression ratio thus nearly the same power
The version I saw on the internet used an adaptor which allowed a spark plug to be fitted in place of the injector, and the engine was run with the COV in the low compression setting (yes really, you can run this high a C/R on producer gas, due to the knock resistance of the Methane, Carbon Monoxide, and Hydrogen consituents of the producer gas)
However the introduction of WMO into the equation, might well produce a gaseous fuel that will not tolerate these CR levels, this will hold true for both spark ignition and pilot diesel ignition, but it would be more tolerable on a spark ignition engine as you would only have to reduce the CR, the only way to control it on a pilot ignition diesel engine would be to carefully control the quantity of WMO admitted to the producer gas plant.
2. you retain the governor so that stable rpm can be assured for power generation
This version was also using the original govenor, but linked to a butterfly flap which formed part of the gas mixing carburettor.
3. you have a seamless transition from dual fuel back to diesel operation and back to dual fuel with no shutdown between needed.
Agreed this is a disadvantage, but since no permanent machining modifications have being carried out, you could easily re-activate the Diesel fuel injection equipment and run it as a straight diesel engine, if you a problem with your gas producer plant.
4. you retain the same ignition timing via injection timing
Agreed this is an area which requires investigation, however the version I saw used a hall sensor ignition system with a wasted spark, ignition timing was easily adjusted by moving the position of a magnet fixed on the rim of the flywheel.
5. much simpler operation, no need for more complicated carb/mixing apparatus.
Nothing complex about using a venturi device from an early diesel engine that had a vacuum operated govenor, even Ford transit vans in the UK are fitted with these devices as part of the EGR cleanup system, combined with a gas mixer ring (freely available in the UK for LPG conversion of car engines), and there you have it a crude gas caburettor.
basically all one needs to do is make the gas, filter it well and fumigate the intake
with a crude mixer and the engine with its governor and injection system take over control of ignition, timing, rpm and fuel offset.
There is the additional problem that producer gas contains a high percentage of Nitrogen and to a lesser extent Carbon Dioxide, so you are introducing gases into the engine which will neither burn nor support combustion, now everyone knows that diesel engine like to operate with excess air. I dont know the answer but I suspect that these gases would behave somewhat like the EGR (exhaust gas recirculation gases) do in a modern diesel engine. They lower combustion chamber temperatures [we all know that running your Lister engine too cool, can cause problems] to reduce Nitrous Oxide emissions.