It wasn't yarn when I brought it home from the thrift store yesterday, but it's yarn now! For the first time ever, I actually managed to unzip the seams instead of picking them apart a stitch at a time. How did that take me so long to figure out? There's still a lot of trial and error, but now I know that it's worth it to hang in there until I find the right bit of yarn.
I measured a stockinette chunk of the sweater -- 4 stitches an inch! -- and swatched, and cast on for Ester earlier this evening.