"proudest spirits of Gre'thor!"

MO: qempa'QeH is indeed a fossilized form, so don't worry about its etymology (though you are correct about no' vs. qempa'). Generally speaking, you wouldn't pluralize qempa'QeH. The word doesn't refer to an individual, but to a unique group, the "proudest spirits of Gre'thor". There aren't two groups of proudest spirits. I suppose you could write a story about the mirror universe where the qempa'QeH of one universe encounter the qempa'QeH of the other, so there would be two qempa'QeHs. In that case, I'd go with -pu'. But otherwise, the plural isn't likely to come up.