What hell you must be going through, my heart goes out to you and your son.
Forgive me but apart from diagnose bumblefoot and provide antibiotics which you were already giving has your vet provided any other help or information?
I don’t think bumblefoot is the problem, the scab is too small to cause the issues shown.
Now the question, if I was you what would I do ?
Search the foot for legions using a magnifying glass if necessary. As it’s nowhere else on her I can only assume that something has penetrated the foot.
Ok, if something is found, what would I do ? Right or wrong I would, personally and I’m not saying his is right but this is what I would do,if I found an entry or wound point, I would open it up. Get out as much of the goo as I could. Wrap with antiseptic cream and put on antibiotics for a week. Separate her into a dog cage and stop her walking. Clean it out and debride every three days hoping everything starts to return to its correct colour and shape.
Ok, it nothing is found, what would I do ?
To be honest I would probably do the same working from the area that looks worse.
She’s clearly in a bad way and I don’t know if anyone else can suggest better alternatives but as far as I can see it, if the infection spreads into the blood that’s it so something needs to be done asap.
I know how this sounds and I’m sorry if others disagree but this is simply what I would do with my 50 years experience without access to a vet but with access to a wise old grandmother,
Sending you the biggest of hugz from south west France