I am ignorant as to how the additional challenges presented by your DD's developmental delays relates to using the toilet, so our experience may be useless, but...
After gently trying a few different ideas: watching videos on using the toilet (there are plenty at your library!), reading books on it, and suggesting it's use daily - all failed - we asked our son when he was going to stop wearing diapers. He said "when I'm four" and on his fourth birthday the diapers were gone and he was on the pot! Had one relapse a month or so later while on a vacation which we handled with the promise of a little ice cream for dessert if the toilet was used, and have never had a problem since. I think giving him the control helped, maybe it would for your little one, too? Best wishes!
-L.