Thanks for the ideas, domreuter!
I hadn't considered the idea of leaving the braces on for retention. That's a very interesting thought that's worth exploring.
My current plan is to order at least one set of Hawleys (possibly two) and one set of Essix for her, once her active treatment is complete. Her orthodontist doesn't care at all for fixed/bonded retainers, and uses them extremely rarely.
Interestingly, we met some friends at a pub last night. They were on their way to a concert at a nearby venue. A friend of our friends was there, a VERY attractive woman of 31 with upper ceramic braces and metal lower. She's scheduled to have them removed in just about two weeks, but mentioned that she'd worn them an extra several months simply because she wasn't 100% satisfied with the alignment and leveling when her ortho suggested removal. So possibly Carrie could go down a similar path with her own braces, and keep them a bit longer.
Carrie is also pushing back her April appointment, which will buy us a little more time. She's definitely interested in extending her treatment time, mentioning it frequently. And overall, we're having a very fine time!