UPDATE:
I encountered a setback just as I was about to mate the bottom cushion with the frame. The one-inch-deep crack between the left bolster and the center section had become a shallow crack barely a quarter-inch deep and the bolster's leather looked flaccid. Porsche's method of attaching the leather at the bottom of the crack was suspect. It involved seating a plastic bead the length of the crack into five plastic clips (see photo below) at the bottom. It looked dubious to me when I took it apart because it depended solely on the claw-like clips. But I shrugged and reassembled it the same way. And I assure you, the bead was seated in the clips, thanks to a tool I built out of a quarter-inch dowel with my Dremel. The seat looked perfect... until the seam's attachment point became detached. I sighed with resignation and then I peeled off the leather that covered the left bolster.
Just as I suspected, part of the bead had pulled out of the clips, but there were also a couple plastic clips, still attached to the bead, that had pulled out of the seat base. The clips are mounted on the inside of the contoured seat base into which the cushion fits. They're called "expanding clips." The bottom part of it is pushed through a hole and then it expands, securing the clip. I left them in place on the seat base when I took it apart.
So I had two points of failure, two problems to solve. Keeping the bead locked in the clips was an easy one: pull-ties to the rescue. But first I had to drill a hole through each clip for the pull-tie to go through (see photo above). A bench vice I picked up at Harbor Freight was indispensable for the task. As for the clips pulling out of the holes, I used tiny dowels (a.k.a. wooden toothpicks) to prevent the expanding part from collapsing and slipping back through the hole.
My workarounds solved the problems. I replaced the bolster's leather, now tight once again, and had a Coors to celebrate.
To be continued...