I see no reason to send the lens back and I doubt anything is massively wrong with it. You've got what, 4 major lens groups to worry about with only 2-3 real ways to screw it up? Seems like you could just exchange elements around until you have "sharp enough" images. Although not razor, the lily looked fine.
An Artar is capable of 100+ LPMM under the right conditions. "Sharp enough" isn't.