How stressful - especially with your deadline. Is there any way around it?
The simple fact is this - if they're not ready, they're not ready. When they get it, they just get it and you're done, you know? My DS was in undies for months before he finally got it (and yes, wee and poo everywhere). I was just about to despair when, just after he turned 3, he just started asking for the toilet.
I personally would not bother with rewards - she needs to want to go to the toilet because she wants to do that (if that makes sense). And to do that she first needs to be physically able to identify the signs that she needs to go.
So I guess you need to first figure out if she knows when she needs to go - then help her from there.