But what is the effect on lifetime of not having any CNO present? If a present-day massive star would have a lifetime of 10 million years, how long would it live if it was a population III star with no CNO?
Carbon is formed in small amounts by normal stellar fusion, and since it is a catalytic process the carbon is conserved. So even an early star would probably have some fusion happening via the CNO cycle.
I'm not sure. I think a lack of metals makes a star less stable and burn out more quickly, but I could be wrong on that. There's some episodes of Astronomy Cast and Ask a Spaceman that I think would answer your question about pop3 stars more reliably than I could.