I know Liz has successfully used zylkene with her ferals and has had some good results. Julie and Liz have lots of experience with ferals. I am a newbie to them and was mainly advised by both.
I trapped a family last year and had two adults and still have the two kittens.
I trapped the four over two weekends so had the adults for a couple of weeks before neutering and about a week after whilst I was trying to find them an outdoor home.
I did end up taking them to the vets in their crate to be neutered because I couldn't get them out and into a carrier!! When the mum was at the vets she get out of her cage, did the wall of death around the room and bit through the nurses gauntlets when she was caught and the poor nurse had to go to the doctors. After that I was even more careful around them!!
Because they were going to an outdoor home I tried not to bother them too much because they weren't very happy around me. I did keep them covered a lot of the time and just kept them quiet. I did put Kalm Aid in their food which my vet recommended instead of Zylkene just to try and keep them calm.
Liz recommended that I tried handfeeding the adults and kittens with chicken roll, this is still a complete hit with the kittens and I had started to win the male adult round with chicken before he went off to his new home. He'd come to the front of the cage and grunt and meow for it then try to bat it out of my hand...bless.
Because she is going to be off to an outdoor home I would just try to keep her as clean, comfortable and well fed/watered. You sound like your doing all the right things and its unlikely you will win her round in the short time you will have her but you could try Zylkene and chicken roll to help her gain your trust.
Couple of important things:
Avoid eye contact and if you do make it, blink slowly and turn away.
Stay low to the ground and small so you don't look threatening
Speak softly and calmly
Very slow movements
Maybe you could leave the radio on quietly just so she gets used to a little bit of background noise