Thanks to all the help I got so far, I can rephrase my first question, being more specific:

- What is the chance of overheating damaging the lower seals of the the wet sleeves of a Nissan VQ30DE engine?

It's been driven without coolant and overheated but did not shut down. After turning it off and letting it cool down, I realized vapor would come out of the exhaust at normal running temp on a warm day, so I had the heads removed.

Thank you all.