Nothing states about HOW you need to do this challenge. So if you wanna sculpt all of those expression, then go ahead i would say. I’m not a rigger but i will go for it.
For example: you could just use a few bones, do a little skinning so that you can open the mouth, lift or lower eyebrows and maybe the nose. Then pose them 15 times as close as it get’s, and then start sculpting on them. Or just start sculpting right away on the base mesh.
Either way: it’s the result that counts.
I think i rigger will have much more work setting up the rig to make it be able to do anything. Even way more then 15 expressions. But posing them will be super fast and flexible.
Sculpting on the other hand for an experienced sculptor, will take little time to sculpt the expression, but more time to do 15 of them. So in all ways i don’t think there is an easy way, but only a choice in path according to your skills.
so relax and enjoy

mops)