Update: It's 11:15pm (I had to break for dinner) and I've just finished knitting the heel flap...the correct way! On the first flap, I K1, Sl1 repeat for each knit row. WELL apparently how you slip the stitch makes a big difference. I had been slipping purlwise...that's ok for the first stitch of each row but you can't do that the entire row! The stitches have to be slipped knitwise! Slipping incorrectly results in a heel flap that's half it's intended length and fabric reminiscent of burlap.
The sock on the left, which will not lie flat because I completed the short rows, has a correctly knit heel flap. The sock on the right is the bad bad bad heel flap. I drew in some lines so you can see where the flaps start and stop.
Also, I know these are terrible photos, but can you see the difference in the texture? The sock on the left has slipped stitches which creates an elongated type stitch and elasticity. The sock on the right has tiny stitches and after I knit the foot, the wrong heel flap was tight and unforgiving.
Thinking back, it's possible I may have even done something like slip every other stitch on the purl row too! I'm glad I ripped back. :)