Currently the atmospheric concentration of CO2 is 396 parts per million by volume (ppmv) and 600 parts per million by mass (ppmm). The total mass of CO2 is 3.06 x 10^18 grammes.
As you say, if we normalise for the number of moles and assume the C is present in calcium carbonate then the chemical reaction can strip out the Ca to leave the C and O with the C oxidising to CO2. So in much the same way as burning 1kg of fuel produces ≈ 3kg of CO2, you’re going to get the same outcome from the CaCO3 and therefore the potential atmospheric mass of CO2 will be significantly more than the mass of C.
The molar mass of CaCO3 is 100 ((1 x 40) + (1 x 12) + (3 x 16)) with the C mass being 12.000%. If your figure of 3.5 x 10^23 grams of carbonate rock on the planet is correct (no reason to suspect it’s not) then C = 4.2 x 10^22g, reacted with O to produce CO2 gives a mass of 1.54 x 10^23g of CO2 (C = 12, CO2 = 44).
1.54 x 10^23 ÷ 3.06 x 10^18 = 5,032.
• 396ppmv x 5.032 = 1,992,941ppmv (of the current atmospheric composition). Or… if all the carbon as CaCO3 were converted to CO2 then the atmosphere would be approx 67% CO2, 26% N, 7% O and 0.3% Ar.
• 600ppmm x 5.032 = 3.019,608ppmm (of the current atmospheric composition). Or… if all the carbon as CaCO3 were converted to CO2 then mass of the atmosphere would quadruple.
Of course, all of this is hypothetical as it isn’t going to happen, and couldn’t realistically happen without some powerful external interstellar or intergalactic influence.
But the point of your question was to establish if the “skeptics” have the ability to evaluate anything about the science related to climate, and clearly some of them don’t. In fact, you only have to look at their past record to see just how little scientific or mathematical evaluation is applied to their statements.
It often seems to be a case of the skeptics saying something because it sounds as if it might oppose the theory of global warming, the fact that in the many cases it’s complete nonsense appears inconsequential. Often the problem is that the skeptics lack the requisite knowledge to realise their own errors. If they had a greater scientific intellect they wouldn’t make so many basic errors. The same applies to the advocates of global warming too, but even a cursory examination of the evidence reveals that this characteristic is predominantly the preserve of the skeptic.