Also I would expect the amount of cosmic radiation bombarding the surface outside of star's envelope would be significant. We on Earth are extremely lucky that

Also I would expect the amount of cosmic radiation bombarding the surface outside of star's envelope would be significant. We on Earth are extremely lucky that we have molten metallic rotating core (possibly done by collision with another planet/moon very long time ago, not 100% sure here), but that's not the default for planets. So yes life can be probably only sub-surface one, but that's generally a tough proposition living off what... geothermal vents and volcanic activity? That's hardly a recipe for anything advanced, regardless of time given to evolve.