Catalysis of the reaction: 4alpha-carboxy-4beta,14alpha-dimethyl-9beta,19-cyclo-5alpha-ergost-24(241)-en-3beta-ol + NAD+ = CO2 + cycloeucalenone + NADH.
4alpha-carboxy-5alpha-cholesta-7,24-dien-3beta-ol dehydrogenase/C4-decarboxylase activity