Hi A.-Marie,
It's great that you realize how important getting enough sleep is for your child especially while still so young! Sounds like she is a great sleeper! We also appreciated the book someone else mentioned by Dr Marc Weissbluth, "Healthy Sleep Habits, Happy Child" so maybe you can get hold of it and see what you think. Sounds like you are very involved parents and like spending time with your daughter - which is as it should be. :)
My next comment may not be well received by many but that's fine - there are many opinions out there, mine is simply one more (no flames necessary! :) ) I'm wondering "Why put her in preschool at all right now?" Sounds like you have a great system at work already, and everything that is "taught" at preschool a child can easily learn at home, with a bit of help here and there from caring adults (like your mother-in-law and you/your h). There is no legal or educational reason your little girl needs to be enrolled, especially at such an early age.
You mentioned you and your h are not generally schedule oriented - I'm sure you've already realized that choosing to go the route of public (or even organized private) schooling is opening your life up to a whole world of outwardly-imposed scheduling for the next 15 years or so from now. Have you ever considered homeschooling? It's definitely not as hard as people often think - especially since you work from home and have a supportive family member involved in caring for your daughter as well (your mother in law) I'd respectfully suggest you might investigate and give it a try. There are tons of resources available for free online and at the public libraries, and lots of active homeschool groups for support and socialization. There are a plethora of activities year round for homeschooled families - social, sports, language, math, science, music, art, travel and more - all in the Bay area alone.
If you're concerned about the quality of education you may be able to give your child - no worries there. Many well supported studies over the years have proven that homeschooled people are much better educated and well adjusted than their counterparts (who didn't have the advantages of home schooling) and they also do much better on average when they attend college.
Successful homeschooling doesn't require a specific "teacher education" or background on the part of the parents, it simply requires a love for spending time with your child and a love for learning coupled with a curiosity for life, which will naturally be communicated to your child. With the advent of the internet, homeschool has become easier than ever before. As your child grows, their learning can naturally become more and more independent, and many HS students finish their "high school" education well ahead of their peers and are then free to attend university or explore other options for their future. I'm very happy to tell you that "preschool" aged homeschooling is the easiest there is - and it's a lot of fun (speaking from experience). :)
I didn't mean to hijack your post about sleep scheduling and make it all about homeschooling! LOL! But I did feel it was a fabulous alternative you may not have considered, and for the good of your child it's definitely worth considering, IMO. :) Then the sleep scheduling wouldn't have to change at all so your original question would be a moot point. Check it out...give yourself and your daughter time. If you decide against it in the end, she can easily begin preschool at the more-usual age of 4, and no harm done by waiting.
Whatever you decide, best wishes to you and your precious daughter!
God bless your family,
S.