Here's a different view: Keep the faulty new lens (from B&H) and send it in to Pentax service under warranty repair if you are satisfied with its optical performance (e.g. lens element centering).
Now my story…
Back in 2012 bought a new K5 and Sigma lens as my first DSLR. About 3 months later I bought a new 50-135mm and FA 43mm from B&H. The 1st copy of the 50-135mm couldn't focus to infinity with my K5 (the pair were at opposite ends of the AF fine adjust tolerances). Exchanged the lens for another copy via B&H's excellent service. The 2nd copy improved the situation somewhat but still not sufficient. Being a major fan of B&H (since 1986) I didn't have the heart to continue dragging them into Pentax's manufacturing problem. So I contacted Pentax service and discussed the situation with them. They advised me to send all three 'under-warranty' Pentax OE items to them (K5, 50-135mm, 43mm). They calibrated the K5 body AF to nominal using a master lens, then adjusted each lens to satisfy AF Fine Adjust = 0. Received the whole kit back in about 3 weeks. Unfortunately, with in a week or two the 50-135mm failed to successfully complete focusing in Live View. So off that lens went to Pentax service again for SDM replacement under warranty.
The outcome…
To this date (knocking on wood
my K5 and its respective lenses have been amazingly accurate in acquiring AF lock. Excluding the effects of the large central sensor size, I have not had difficulties with front or rear focus and very rarely have experienced focus hunting. Of course this outcome was the result of system calibration rather than SDM repair and the K3 already performs more reliably.