they have tested one to F250 (or thereabouts) at which point one of the engines stopped, presumably because the ECU could no longer work out what was going on. It restarted quite happily at lower altitude and there was no permanent damage.
What is the current certified ceiling with the 2L engines?
I would be a little concerned about engines simply suddenly stopping due to altitude. One might have to do an emergency climb one day... Still, FL180 is OK for an IFR tourer, just.
On downtime, pilots' attitude varies enormously. Many evidently accept periods of months of no flying.