Have you tried a seperate supply of known good fuel?
I wonder if the engine may have run on or coolant or excessive carbon in the cylinders this could cause multiple injector tip damage. Worth checking in the intake system for any evidence of this. It may be worth having a look at the glow plug tips or you could try removing an injector and connecting it to the rail so you can see the injector then carefully aim it towards a container and see what is actually happening when trying to start it.