How does she do on OT? E isn't quite as sensitive to it as some, so if she was doing what D is doing I might be tempted to say - one nap and that's it. Put her down at 12, bedtime at 6/6.30 and fight through until that sticks. As Anna says, they do tend to extend their nap once they only have one, and the problem at the moment might be that she is just a bit confused as to what she is meant to be doing.
(((hugs))) Hope she's not a PITA for you today.