Maybe. This is going to be a tough case for the prosecution. They have to have her testimony to have a chance. They just have to hope the jury looks past her shortcomings and latches on to what they need from her.
Yep. They don't have much choice but to let her testify that Zimmerman approached Trayvon first. It's about all they have besides a he said/she said argument where one is alive and the other is dead.
This is a political prosecution meant to appease voters. There is no way the DA brings this to trial in a normal case with such little evidence on their side and their only significant witness already admitting to perjury. To get a unanimous guilty verdict, or even 5 out of 6 would be close to a miracle, but the DA can at least say "I tried."