Another option may be to keep the pups below "breed ring" weight, like the dogs were in the 70s and 80s. Granted, that may not allow full genetic expression of the joint issues, but it is one environmental thing that can be controlled while one breeds into a strong joint line, with vertical AND horizontal pedigrees analyzed. I am not sure that all Labs don't carry for HD, if one packs on the pounds and exercises like a weekend warrior, the way some pet people do. Additionally, one may want to use a line that is also Penn Hipped with good results.
First, I would have the bitch x-rayed again to see if there is DJD in the elbows. I have seen some dogs that had clear elbows when younger, but the elbows degenerated with age. Those dogs were re-evaluated because their production record was not good. I would say the same about your girl.
I have had the same thing happen with hips - clear at age 2 and dysplastic at age 4. The proof is in the whelping box. If the dog is producing a problem, then it is time to re-evaluate or remove it from your plans.
