Hello!
I tried to verify your uECC_point_mult() routine (using secp256r1) against the test vectors at http://point-at-infinity.org/ecc/nisttv.
For k=1, k=n-1 and k=n-2 the routine generated the zero point. Other values of k seemed to generate correct points.
I also tried to disable regularize_k (setting the bit count to numBits(k) as early versions of micro-ecc did) and noticed something different. For k=1 the routine generated the same point as it should for k=3. For k=n-1 it generated the zero point. Oher values of k were ok.
Is this intrinsic to the algorithms you used, or perhaps some code is faulty?