The "unknown valve" is a "lean cruise power valve" which reduces gas flow when manifold vacuum is high. It should not prevent starting as there is always a gap through it. Adjustment is via a screw on one side and a nut on a thread in the small cap on the other side. With the vacuum hose off and capped it should give full mixture.
I'd be pulling the lid off the mixer (5 screws) and looking for a torn diaphragm. Don't lose the spring under the lid. A tear in the diaphragm will prevent it lifting and allowing any air into the engine.
The ECU does exactly one thing, adjust the fuel injectors on time. It won't affect the LPG.
Last thing, pull all the plugs out and dose each cylinder with some penetrating oil to make the rings happy, Crank it over to distribute the oil, then fit a new set of Bosch WR7DC plugs. I've experienced a number of sets of plugs over the years that just didn't want to start, replacement and away it went.
Oh and go easy on the petrol spray, these things are easy to flood. Starting fluids are better, e.g. start-you-bastard.
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