Okay, it seems that 'work 57 rows in patt dec 1 st at each end of every 14th row' means concurrently, not consecutively. I would have known this without thinking ten years ago, but now... and after a 18 months of editing and mentally putting periods into the middle of run-on sentences... well.
I've ripped out the 19.5" I knitted this week and have cast on again and worked up to the first decrease row. It's not like I can write this weekend anyway, because I have a friend visiting and I'm not going to be burying myself in my laptop when I only get a few days to fit in six months' worth of chat, right? Plus, if I'm working concurrently, the sweater will take half as long to make as I thought. So I can still be done by next weekend, maybe.
I'm going to start writing again on Monday, though. Two weeks off is plenty.