it's wonderful that you've been able to rescue this poor budgie.
I don't have experience with a budgie like this and I've never been concerned myself about finger taming my budgies. I let them do as they want, so I can't offer you any advice that will be helpful in those terms.
What I can say is that in regards to her fear of toys and treats, you could try and leave them on the outside of the cage first. If that still seems to upset her, it might even be worth putting them where she can see them, but where it's not attached to her cage, just near her cage for her to observe.
Many birds have a fear of new items in their cage who don't have a bad past so it's not completely unexpected for this to be happening.
As for the fear of the dark, she may have had a night fright and is currently uncomfortable in the dark. It might have happened during the night. There isn't necessarily something that causes it that can be dealt with.
I had a period of time where my birds were having night frights every night and I would leave them uncovered with a night light on and they eventually settled back into a normal sleep routine.If it takes leaving a lamp on for a while, try that for a while and try and reduce the amount of light. Do you maybe have a lamp with a dimmer switch that you could reduce over a few nights?
Maybe it's the sudden change from light to dark that unsettles her?
Perhaps try and let the room naturally grow dim and see if that helps?
As she's so nervous, have you considered keeping 3 sides of her cage covered at all times? It might help her to feel more secure.
I'd also back off from all training with her.
I wouldn't currently try to finger train her if she's so afraid of hands.
I'd work to let her feel safer in her new home. It sounds like you were making great progress with her and that she's happy with you. It might take a lot longer to get her even slightly comfortable with you.
I would try and tame her the way one would tame a new baby budgie and just work with getting her comfortable with your hands touching the outside of the cage. Then progress to your hands being in the cage, and her not reacting in fear.
Others might have some more experience with this, but I think you're doing a great job so far.