Hi Mama,
It does sound to me like things became a power struggle during the last round. I'm inserting a link that's meant for early childhood ed students, for their work with youngsters and their parents. I have found this child-led approach works well for me and the children I have cared for.
http://www.betterkidcare.psu.edu/AngelUnits/OneHour/Toile...
Just my two cents here: I've found over the time I've worked with kids that children can do well with incentives for a while, but they aren't infallible. Some children who didn't really need the incentive will do great with them, and be fine when they stop. Other children will tire of the incentive and stop using the toilet again.
When I was nannying, and with my own son, I really listened to what the children were wanting. So often, when they are saying "diapers" to us, it's not always to *fight* us on it. What they are saying is "this is more than what I can do right now". Some kids are in underwear for a while, then ask to go back to diapers within a few months of a big change: moving, new sibling, starting preschool, family changes. This sort of regression is common.
If it were me, personally, I'd wait until she's interested. This might happens sooner than later, especially if her peers are going to the bathroom on the toilet. Girls are typically more socially oriented and like to be part of the group. You might find that just waiting a little while and dropping the subject entirely will do the trick. They're only ready when they're ready!