While Cushings Disease would have meant access to supportive drugs, this is still a good thing, the vet assures me.
Her normal insulin levels mean we've been managing her diet well and the vet said that Equine Metabolic/IR horses can have normal levels so that's still on the table.
At this time, I have no plans to have her tested for EMS/IR because the tests are prohibitively expensive (I have fancy new shoes I have to buy Bud every four weeks), can often miss the IR and even if she is diagnosed, we wouldn't be doing anything different than what we are already doing. There is no cure or treatment for EMS, other than diet management.
Buttercup hasn't improved much with her new shoes, but she does seem more comfortable from last week.
Here's the latest lameness video, which I've sent to person who is excellent at analyzing lameness to see what he thinks: