I watched it from the link on TOB. I will have to rewatch it and write down the 24 behaviors...so I can watch for them. For me, it is pretty easy to know (like you) when something just is not right. Finding a vet that will take the time and believe is sometimes another story.
I had a mare that I started without so much as a tail swish. By the time she was 6, she was balking, kicking out at the leg and just generally miserable.
It took me about 3 months to realize it was cyclical

. I took her to the vet and had a repro exam and they found nothing amiss except multiple follicles on one ovary...at least that is what it looked like. That was late August and I had hurt my back and was not able to ride. I let her sit until late December and we checked her again expecting it to be back to anestrus size. Hmmm, left ovary was still at least 3x as big as it should be. During the fall, her poor pasture mate was savaged by her a couple times...luckily, there was a fence between them. She was pissed and it was all his fault. I wanted her spayed and the vet said...we don't do that (this was like 30 years ago before endoscopes). He referred us to another local vet hospital with surgery capabilities and that vet agreed to do it but was mumbling about "these women and their horses' female problems"...something to that effect. He did the surgery and she had a solid tumor on the left ovary which made it like lemon sized. It wasn't a hormone producing tumor but I am sure when she was forming follicles on the non diseased part of the ovary, due to the mass she hurt like hell.
Granted, that wasn't a lameness but my trainer at the time and the clinician he worked with just chalked it up to her being a pissy little mare
I started this horse and never had any issues like this. Something had to be wrong and it was. The behavior was totally due to pain. The spay surgery worked. She was guarded and tried kicking out about 3 times after she went back to work until she figured out...oh, I don't hurt anymore.
Susan