Fortunately she replied very promptly. Unfortunately, my error was at the very start of the pattern, 12" back!!! The armhole meassurements were supposed to be taken from the end of the short row shoulder shaping, not from the cast-on edge. Somewhat ambiguous directions in the pattern, so I can understand why I got it wrong, but it's probably one of those things that should have been obvious.
I was not going to rip back 12" (plus 2 pockets), so I decided to take a deep breath and try my hand at sweater surgery... figuring that if it went horribly wrong, I'd just rip back after all, and only be a little worse off, than if I'd ripped back right away. I've only done the left front so far, and am reasonably satisfied with how it went, even if it did take forever. I just hope it won't be too noticeable that once stripe is suddenly 1" longer than the others.
While the cardigan was in time-out, I picked my Professor McGonagall blanket back up, and actually managed to finish the colourwork last night! Now I just need to make the border, and I'll be done!