After reflecting upon the situation this afternoon, I have come to some conclusions.
First, I think that the blame for my error lies firmly upon the shoulders of my Green Woman.
Why would she do such a thing? While it is true that she is often mercurial, frequently chaotic, and usually serendipitous, the Green Woman usually does have a method to her madness. She clearly had her reasons, and I think I've figured out the two that could have prompted her to meddle with my knitting. She either:
A. Wanted me to finish up the tiny cabled sweater I began for the Pixie several months ago. She is, after all, very fond of the pumpkin colored yarn, and she delights in small sweaters. Plus, the cold weather we've had this week may have caused her to worry a bit about the warmth of my wee girl. She loves both of my girls beyond measure, and has always considered them both to be a wee bit fey.
B. Wasn't too happy with the fact that I just wanted an easy knit...you know, the sort of knit where you don't have to think because you can blindly follow the pattern. She is wicked voice in my head that usually prompts me to pick out the most difficult path or pattern, and she scoffs at projects when the words 'easy' or 'simple' are attached. Personally, I think she thinks it's fun to watch me fuss, fiddle, stress, and obsess over the details since she has never done any of that about anything.
Second, this really is the sweater that I want to use this yarn for. I did consider scrapping the whole thing...for about two seconds. It's going to be lovely, and it's definitely worth taking the time to do it right.
Finally, if I'm going to be knitting for myself I need to give my own projects the patience that I give the work I do for everyone else. I have no compunction about ripping and redoing (for example) a sweater for my husband three or four times because I love him so much that I want it to be as perfect as possible. I deserve the same.
I'm going to rip it out tonight, and give myself a couple of days to recover from the shock and redo my math. In the meantime, I have a wee, pumpkin-colored sweater that just needs two sleeves.