Interesting. My new baby (84 300D, 199,000) acts in the same way in regards to the “(strange?)” cooling system pressure characteristics that you mention.
When the engine is warmed up, the level in the expansion tank drops – I was assuming that it is due to the expansion of the hoses & resultant increase in their volume/capacity.
However, like yours, it holds pressure overnight; the hoses are still firm (pressurized) in the morning. If I open the expansion tank cap in the morning, the system gurgles quite a bit & the level in the tank goes back up to normal. My other (gasoline engine) cars do not do this, so I am concerned about this being ‘normal’ or not. I hope that someone can chime in on this?
By the way, I am not sure if the 617.95x engines use stretch bolts or not. But remember that IF they do, stretch bolts cannot be reused, they must be replaced. As far as re-torquing the head to eliminate a leak, depending upon the circumstances, it MAY work temporarily, but I wouldn’t consider it a proper cure even if it does work.