Hi there,
Sorry for what you're going through. You've had good advice on here and very questionable advice from the vet and rescue lady.
I so hope you are able to work through this and don't go the rehoming route. I would probably have rehomed Mollie once a week by now (she's black BTW) for doing what Ginny's doing, but I never felt like I could pass the problem on. (Our 2 dogs, one of whom is a worker and one a worker/show x, are the same age, and they fought regularly for a year or more until Alfie figured out who's the boss and that it's not him.) Mollie will also kick off with some strange dogs she meets but this is mostly because they are overly rude and get in her face or bum, and she won't tolerate it. Is Ginny just tellling these dogs off for rude behaviour?
I was actually glad of Mollie's fiery nature a few weeks ago because a husky started attacking Alfie quite badly in the woods. Mollie rushed in to defend him and actually chased the husky off. It's not often I've seen an upside to her in this respect.
Vet check, behaviourist if necessary, and if no blood is drawn, you may well be able to manage the situation well. You may always have to "manage" it but I wouldn't give up. Good luck.