My kids have always stayed up much later at disney, thankfully they will sleep late too. We have done a few christmas partys, and halloween partys, and we have on more than one occasion been leaving the park very late with them. Sometimes they have crashed in the stoller early, (or one or two of them...we have three). They will sleep late in the morning (sometime after 11), and since we go for two weeks, I don't feel the need to get up early every morning to be in the parks at rope drop. (plus we go off season) When I know we are getting up early, I try to plan an early night the night before. But, my kids have traveled a lot, and are used to being very flexible when it comes to vacation.
I say, see how it goes, try it one night, if she falls asleep in the stoller, or is happy as could be stay out one or two nights. You don't have to stay every night, but I would of been very disapointed if for two weeks I had to sit in my hotel room from 630 at night on.
My kids have had a meltdown or two, but then again so have I. In september when we were there, we were not used to the heat, and that got us good, thank goodness the parks were empty, because we spent lots of time in the pool, but still fit in everything we wanted to.
I would say see how things go, don't push it, but try to see what happens. They are so easy at that age, your baby could fall asleep in the stoller, and you could have a few great hours of just enjoying things while the baby sleeps!
Good luck with whatever you decide!