“Outside air is much thinner than we need. Using it would be VERY expensive!”
Actually, the air gets compressed by the engines, and then some of it is diverted to the air conditioning/pressurization systems (called ‘bleed air’). It is both hot and compressed...but it is air meant for combustion (and thrust), so there is a fuel consumption hit - maybe 5% of simply recirculating air (but I don’t know for sure).
It is doable...and should be considered, since airlines will soon be fighting for their survival.
IDK.
Seems that air is intended for other purposes and would be very expensive to divert.
But I appreciate that you’ve thought the idea through.