ḱr̥h₂os-

I don't see why. They're different roots, even if they might be related.

Which descendants point to Reconstruction:Proto-Indo-European/ḱerh₂s-? Only Germanic and Latin cerebrum?

I guess so? But the question, perhaps, is whether this is a new root or merely an existing root with some kind of suffix.

Nussbaum has a whole book dedicated to this root. I haven't read it. Let's keep everything on one page until someone sorts things out.