Is there a higher variance in HU cash games?

Where is variance reduced to a minimal amount? I think that 9-10 handed there's less variance, and in 6 handed there's more variance than 9-10 handed, and that HU is where variance is the highest. Is this thought correct?

Generally speaking yes.

I think also the answer is yes. And you are right about 9-10 handed less variance than 6 handed, also Hyper Turbos are the most variance games.

Yes absolutely.