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Forgot to add:
Coolant recovery tank was bubbling like a fish tank (~3 big bubbles per second), and the plugs all came out intact with the center electrodes all looking just fine, BUT - there was whitish residue on the “elbows” … but we already know there’s a coolant leak, so these symptoms just verify that, correct?
Compression readings on the gauge dropping after one hour aren’t indicative of anything in the engine. It’s just the gauge. There’s a valve core in the gauge that keeps the pressure in the gauge, like the valve core in a tire valve stem.
Your compression readings are perfect. Your coolant leak is external, from your picture and what you describe I still think it’s the timing cover gasket. Remove the coolant overflow tank, pressure test the cooling system, get a good flashlight and mirror and take a closer look.