I would not worry about the temp, if you're seeing 100C cruising at 70mph/90* ambient, especially if that's with the A/C on. Have you verified the coolant temperature or is it possible that the gauge/sender is/are off slightly? That is not really very hot for a warm day and it sounds like it idles well and recovers quickly from a hot run. A possible indicator that it simply has a "happy gauge".

It is possible that the water pump has rotted away to a point where it isn't moving enough coolant (see cavitation pitting below), but again the quick recovery and reasonable idle performance indicates a healthy system, maybe there's deer fur behind the condensor or a weak fan clutch. Running the A/C automatically runs the electric fans on low so that would mask a bad fan clutch during your idle test, high temperature also runs the fans (high speed) and would bring the temp down quickly. Do you ever hear the roar of the clutch fan?

As far as green coolant, no BFG unless it is not a phosphate-free quality green coolant, it's just the color. Change it to the proper coolant, green does not hurt gaskets and radiators, aluminum-engine and aluminum-radiator/heater (green) coolant has been around for over a quarter-century. The more important part is the cavitation resistance, especially in a diesel engine, that's what the G-05 and other proper HOAT diesel formulations bring to the table, a good quality diesel formulation changed (or balanced) regularly will not hurt the radiator. The bigger problem would be cavitation-pitting from the wrong coolant (per Mercedes-Benz literature et al). Change the coolant to M-B or Zerex G-05 and don't worry about the old coolant color any more.

Running a lower pressure could lead to localized boiling, which can cause problems also. I would say that your better bet is to run the proper system pressure and keep adding coolant if you're going to run it as-is.

Not holding pressure overnight, what does that test again? You've already established that you have a leak so, ...
Personally I think that there are too many ways that test can be wrong. A better method IMO is to start the engine cold, and feel for pressure building quickly; before the engine is warm. Or have it tested for the presence of exhaust in the coolant (many shops can do this).

There is no way that you can guarantee the car will not catastrophically lose the head gasket in its present condition, nor with a new head gasket. If dependability is your goal then you're going to have to ante-up, and what about the vacuum-pump and timing chain, the belt tensioner, the transmission, ... ironically I've been left more times with new cars in the first 2years than any of my older cars.

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Gone to the dark side

- Jeff