if you have coolant getting into the supply hose and mixer then the vaporizer is bad, the diaphragm on the coolant side is leaking probably from deterioration.
Also you should remove the solenoid valve and energize it while disconnected to see if its even opening up to allow LP liquid through into the vaporizer. (if dirty or worn out they tend to not work well and will stick)
NOTE: this will be under very high pressure so be extremely careful when performing this test and keep it out of line of anything like your face or hands, just have a firm grasp on it when you open it up to check for flow and only do it for a second. YOU WILL know immediately if its working or not ;o)

Once you confirm the shutoff valve is working and you know you have pressure on the inlet side of the vaporizer you should get a regulated pressure on the outlet side consistently. On these old systems there was no filter lock-off or vacum lines running to the side of the vaporizer to open or close the flow, its either on or off depending on the shutoff solenoid valve. What controls the flow into the engine is the mixer diaphragm operating under the regulated pressure out of the vaporizer.

As for the mixer , if it is contaminated badly as you say... clean it out, check the diaphragm and make sure its not torn or deteriorated and leaking, if it is damaged it will not pull open when the engine is cranked and the engine will not run. Plus in the middle of the mixer diaphragm is a chamber with a small bullet looking thing, make sure this is clean, this is actually a valve that seals off under spring pressure only opening under the vacum from the engine and if it has any leakage from being dirty the engine will run badly or not run at all.

On the compression issue in the cylinders, this may improve as oil gets into the rings because if it has sat for a long period the rings tend to stick and you will loose compression at first. Once they loosen up this should improve, but at the compression levels you say you have the engine should run.