Mehrn's using her hands like people use prayer beads, or an abacus. It's not magic, or even necessary, but it helps her focus. She closes her eyes for the same reason.

Yes, they have more or less two thumbs. So do koalas. It's not unheard of.

That all-teeth expression isn't a social smile, it's a hunting expression. Most grins in the world are predatory. Humans are weird: "I like you/am amused, let me illustrate my innate capacity for ripping your flesh!" Maybe it's because our teeth are so pansy. "I see you also are a soft-skinned clawless weakling! Let us expose our total lack of natural weaponry by baring our tiny rounded fangs!"

I wonder if that confused other animals, when humans started spreading out. "Geez, look at that hairless ground sloth, trying to scare me with its pitiful little nut biter teeth. Easy dinne--owowow why is there a sharp thing in my eye? Am I on fire? Screw this!"

And then fifty more grinning freaks dropped on the cave bear.

Maybe our smiles are hunting expressions after all, but with the human tendency for mental time travel added. "Hi! Remember when we wiped out the cave bears? That was great! Let's do it some more!"