Those are all good questions I don't answers to but, could you expand more on your last sentence.
A crack in the cylinder head, between the coolant jacket and the exhaust side of the head, could lead to "coolant in the exhaust" which is what the mechanic said was found. This is also what appears to have happened in the thread to which I provided a link earlier.
If there is a crack allowing coolant to get into the exhaust (subject to clarification as to where), the overheating could be caused by the loss of coolant, or perhaps the effect the coolant in the exhaust has on the air/fuel and O2 sensors at the catalytic converter. That was why I asked for more information on the driving situation when the car overheated.
Also, in many if not most threads on overheating, the symptom appears under specific conditions -- it might be while idling, or while ascending a long, steep slope, or while driving down an interstate at a constant high speed, etc. Sometimes, this information can be used to narrow down causes.
Information, indeed, details, that might be overlooked can be quick leads to a solution. That's also why I asked about the fans; do they come on when the temperature gauge goes above normal? They should, but if they don't, that itself could be the cause of the overheating.
The point is there's many possible causes of overheating, and we need to look at the possibilities to try to diagnose from a distance. When someone observes coolant in the exhaust, that should be considered, at least, delved into in order to understand exactly what it refers to. It might not be something to discount summarily. That's why I said
I wonder if there's more to this statement [about coolant in the exhaust] that might be related to the apparent overheating.
Hope this helps clarify . . .