You cannot imagine how sorry I feel for you and your poor dog.
I understand you so well as I had to go through the "cast topic"
already three times ( two broken legs and one torn ligament). It's
a really frustrating situation, you count the days till the cast will be
taken off and then in your case the foot has gotten worse, that's
incredibly depressing. I keep you in my thoughts. When my dogs
had the cast, I took them out every day, they hopped with three legs
and even got used to it, as the bandaged leg was immobilized they
couldn't use it. We went for 20 minutes in the morning and another
20 minutes in the afternoon, they even pulled on the lead with three
legs. Of course they went on the sofa with their cast, animals adapt
amazingly to those situations. I think if the foot got worse it must be
the fault of your vet. Maybe I would try to get another opinion.
My very best wishes for you, keep us posted.