J.D.
I have two sons, one of which potty trained at 3 1/2 and the other completely by 4 1/4. Unfortunately, I tried everything under the sun, including things you shouldn't do out of desperation, and really nothing worked until they decided to do it. It may help to enlist the help of a neighbor as well as daycare. It may also help to arrange playdates with friends who already use the potty regularly and wear underwear. Give her alot to drink beforehand too! Positive peer pressure has been known to help.
I didn't know to try this, but doctors say always change the underpants in the bathroom and deposit as much of it as you can in the toilet so she sees where it goes. You may want to explain that too in case she is afraid a piece of her is lost forever.
I would say for your sanity, take the pressure off. I would say the only time to insist on her sitting on the potty is before you go on outings when accidents will be a huge pain. The rule could be that everyone go to the bathroom before you leave to go anywhere. Whether you need to go or not. Even if she just sits there for a few seconds. So she sees you go, everyone else in the family go and also her. (You may want to put her in pullups then for the ride.) My boys got into the habit of going to the bathroom before we go anywhere. I kept a potty in the back of the van for a couple of years for those just in case moments. It helps prevent accidents and having to go to some dirty public bathroom. Maybe if she gets in the habit of sitting before she goes out, something will happen!