I know with your extensive effort, it's not this, but just to add to the conversation... What ratio antifreeze/water are you at? I had similar issues on a Perkins diesel in my tractor. The float/ball type indicators are worthless. I had way too much antifreeze. I got a refractometer type...
I can't really see what you're talking about in the picture, but years ago, I had same/similar problem. Replaced o-rings, no issues since then. I'm sure there's videos somewhere on the internet. I seem to remember finding the correct size/type o-rings was the hard part.
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