Thanks, all, for your comments on my earlier post about course load and work and life and etc. Your thoughts were helpful, and inspired me to do the obvious thing: email my advisor. I asked him about my course load, whether it's "typical", for whatever value of typical that might be, and for his thoughts more generally. The upshot of his reponse was that some people have done the degree in two semesters, but longer is more common, and what I want to do is totally up to me but what I have in mind isn't completely whacked.
That, of course, allowed me to give myself permission to drop one of my classes (or, more accurately, not to petition to get into an already full class that I desperately want to take, but which will be offered again next term) in favor of taking two credits over the summer, still finishing the degree in a year -- just, not a school year.