I feel your pain. My daughter was very difficult to potty train. The good news is that she did not start college in diapers. We tried absolutely everything suggested here (including naked butts) and elsewhere and what finally worked is that SHE became ready and wanted it for herself. Not us. Not her pre-school. Just her. She had no interest in stickers, or pleasing me or getting treats or toys. She was poop trained early at 2, but pee took until she was nearly 5. Being wet was really not a big deal for her and she was far too busy to stop what she was doing to use the bathroom. Plus, she was VERY strong willed and I think part of the delay was pushback from her. She *knew* what she needed to do, she just wasn't interested in doing it.
The fact is, that you cannot control another's bodily functions. I backed off and once she was good and ready, it finally happened. I think the idea of starting kindergarten was the impetus. I'm not sure what would have happened if her preschool wouldn't take her in pull ups.