As of yesterday morning, I still found the weaving part of the interlaced insertion stitch pretty terrifying, but I think I've worked it out. This seam has been a learning process, but as the embroiderer of Birgitta's Cap missed stitches, too, I didn't feel too much pressure to create perfection. This was compounded by the fact that no set of instructions that I could find for creating the interlace part of the insertion stitch ever showed what to do if you had more than two intersections to weave around, so I was winging it a lot of the time. Assuming that the goal was to create an over-under woven structure, I think I did pretty well. It's not always even, as I would sometimes discover partway down a row that I'd not been picking up an "under" stitch from the back of the woven structure, but right at the top of my head I think it all looks best--most even, most complete interlacing.
Oh, and lick your thread. Seriously. Linen frays as you work with it, and if you wet it like you do when you're spinning, it maintains cohesion much better than otherwise. I wish I'd known this while I was working the initial herringbone stitch, as it'd have saved me some worry and heartache over the integrity of the finished piece.
( I would never recommend this stitch for beginning embroiderers, but brave intermediates, maybe. )