Eleanor Kellon, VMD
Both her insulin and her post TRH ACTH were way over the top. The uncontrolled PPID is likely driving her insulin. It's not only cortisol/ACTH that causes this in PPID horses. The other hormones do too.
You are absolutely right that phenylbutazone typically does not do much for laminitis pain and she may very well have arthritis. More comfortable on extension than flexion suggests knee or fetlock but you would need a good lameness exam with nerve blocks to pinpoint pain locations. Even if she does have arthritis pain, that's no reason to ignore her metabolic issues or you will be adding more foot pain to the mix.
Eleanor in PA