I think you're getting good advice. I would definitely not wait the 28 days before trying to get the scab off. It won't go away on antibiotics alone. Definitely start soaking it twice a day and try for the scab in a few days or so. Just slowly start loosening it with your nail like it was mentioned. I'm not sure what it will look like under there as it's so large. You should look through the threads on BYC for chickens and ducks with bumblefoot, as I know there are some that post pictures when they do the "surgery" to get the infection out. I've seen some pics of chickens with pretty large bumble scabs. Some people seem able to get a kernel of infection that pops out like Miss Lydia's and sometimes it seems that it takes going in a few different times over the course of several days. Recently one of my ducks had a little bumble and I thought I did get the infection out as soon as I noticed it - it was just a small swelling by her toe. But it seemed like she wasn't getting better, even after a trip to the vet and 10 days of antibiotics. Back she went for an antibiotic injection but the toe swelled even more the following week, though the initial bumble spot by the toe flattened out and seemed healed. Third trip to the vet and he said it had abscessed and gotten into the toe joint. I ended up euthanizing her because he said the treatment would have needed to be: x-ray, needle culture, culture analysis, 8-10 weeks of antibiotics, and he said it was very possible it could lead to amputation. I opted not to put my duck or myself through that regiment. I will attach a photo of what her foot looked like at the end - it doesn't look bad, but the infection had spread, despite our best efforts.
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it must have been so hard for you im very worried this hasnt been treated by the vets correctly to begin with 
