Oh no, I was hoping to come in here and see a much more positive update.
I definitely think the next step should be to call the surgeon who did her LP surgery, even though I agree with your thinking she would be limping or showing reluctance to walk on the leg if the pin broke. Maybe he will think of something your other vet has not come up with.
As far as gland issues go, my last dog had anal gland issues for awhile and he definitely did the "scoot" thing as well as let out a really foul smelling odor.
We had his glands expressed routinely and eventually the problem seemed to clear up on it's own. He had always had a diet high in fiber, but the vet said the placement of his glands was odd making them hard for him to empty himself when he pooped...which to my way of thinking should have meant he'd always have the problem.
I just watched the video, and the panting and shaking both signal pain in my mind.
I'm so sorry both you and Tatum (and the whole family) is going through this.
ETA: I just had one thought. Are you both gone during the day or are the pups alone at anytime where something really random could have scared her without your knowledge? I'm assuming it doesn't matter what room she is in in the house she still does the shaking and panting once the "episode" starts, but I figured I'd mention it.