With regards to the harness and managing to get it on.
I would play a sort of desensitization "game" with her. If you clicker train I would do it as follows:

Sit down next to the dog with the harness in your hand, a pot of really high value treats and your clicker. Push the harness out across the floor, click, treat, big praise, push it a little closer to her, click, reward, push it closer to her, click, reward, ask her to do a hand touch (touching your palm with her nose), click, reward, ask her to target the harness, click, reward, move it closer to her, click, reward etc...

You don't even need to put it on her. But you want her to be doing little tasks involving the harness (targeting etc), so that she has to work for the click and reward. The idea is that once she starts focussing on working for the click, the fear of the harness (because she associates it with going out and fireworks etc...) shouldn't be so prominant. Hopefully she should make more positive associations with it too.
Obviously though she'd need to know how to hand touch and target objects on command, so if she doesn't know them yet you'd have to train those behaviours.

Hope that helps

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