In less than a year, too! My speed seems to be improving!
That's 36 total inches, accomplished during the few bits of motivated spare time I've had since mid-January. Next on the docket is to trim off all the extra linen, give it the tiniest possible rolled hem I can muster, and do some simple needlelace on the ends.
My previous screwup with the design ended up being perfect for learning and practicing a stitch that would work for the edges. I got the basic needlelace stitch (itself a variation of a blanket stitch) from this tutorial but added my own extra step to give it the bumpiness that exists in Holbein's painting. It's maybe not quite exactly the same, but it seems really close and plausible.
The neckline embroidery may very well end up going much faster since the design is pretty basic, but it's something I'll have to chart out myself and execute in white...so maybe sixes after all is said and done.