Used oil analysis is very effective as well. Even if there isn't visible evidence of coolant in the oil the appropriate elemental analysis will be off the charts.
There are times the way the gasket fails it allows communication between the coolant passages and the oil drains from the valve area back to the lower crank case, or one of the high pressure oil supplies to the head, but not coolant jacket into the cylinder.. or at a much lower rate.
As...
I hope they take care of you, and please keep us posted on what they find, as rare as problems with these are any information is valuable for the knowledge base.
Do you often check the fluid levels? Coolant bottle empty at all?
Ask them if there is evidence of the valley plate coolant leak. That isn’t uncommon and can definitely lead to low coolant and eventual HG issues.
Unless I’m reading things wrong you got a misfire out of the blue and that was...