Would it be correct to say that at a higher Mach number one would for the same mass and altitude require a lower Cl to maintain 1 g flight and therefore the limiting factor is in fact thrust?
Absolutely correct. That's the gist of high altitude flying.
With more thrust at the same altitude a higher Mach number could be obtained and thus a lower Cl.
Yes, but at our lift ceiling, we
are at Clmax. No level flight is achievable, whatever the thrust.
Could recovery from this ceiling not be safely be obtained by lowering the nose and making an accelerating descent. As in doing so Cl is reduced and therefore high speed margin increased?
Yes, as immediately upon lowering the nose - and begining the descent-, one is in fact reducing the AoA, hence the Cl and therefore escape the situation. Acceleration comes a bit later, as we see the buffet margins widening with diminishing altitude.
Google a very simple paper made by Airbus with "Getting to grips with aircraft performance". A very nice paper for reminding oneself of basic theory.