Using a tapestry needle threaded with crochet cotton, I added a lifeline at the point where the sock leg divides for the heel and the instep (the instep stitches were already on a lifeline)...
... carefully unpicked the just-grafted heel...
... and frogged it. (The leg looks so... incomplete without a heel.)
The lifeline not only prevented frogging from going beyond the selected round, it also made picking up stitches for the heel flap much easier.
The #126 colorway of Trekking seems to have less sproing than other colorways and the knit fabric seems unusually dense and strong. I think this will be a hard-wearing pair of socks. The extra ease in the new flap-and-gusset heel makes for a much better fit.
As a bonus, what would have been a brown bullseye ring around the heel turned into a nifty brown patch at the heel turn.
Unplanned, happy serendipity! Tra-la-la-ing has resumed.