re: Yeah, if it takes a billion years to do one cycle
I don't see why centuries for a complete cycle is a slam-dunk against microbial life replicating and diffusing to other convection cells faster than it's destroyed. Maybe if there's very little leakage between convection cells, I suppose.
Centuries seems likely to be a slam-dunk against life originating there, however. I'd think life would need a lot longer to get started, especially when it has no solid substrate to develop on.
Who knows though, we have a sample size of one when it comes to life.