Without copy and pasting large chunks of the foregoing, I don't completely accept that the circumstances are entirely as prestented.
It is generally accepted that engine time counted from the engine overhaul interval is time accumulated at a fairly high power setting. For most single engine aircraft, the tachometer "hour meter" is actually an odometer of the RPM, not a clock. So if you ilde for hours, you will not accumulate hours off the engine life. (Doesn't do the engines any good at all though)
If a person ground ran engines at such a high power setting, and for so long on one occasion, so as to noticably decrease the fuel quantity, I would expect the engines to be quite overheated, and probably damaged. If I, as the pilot to follow, thought an aircraft had been ground run that long or hard, I'd be much more concerned about the condition of the engines for the next flight, rather than fuel used. But I would of course check fuel anyway - I've learned!