I will have to be the one to disagree.. I have seen a horse fake a limp in a front foot. She was well over healing from her abcess (yes, she did have a problem before), but in front of the guy who owned her she would limp. The second he turned away from her she would be just fine.
Your mare though sounds like she does have something else going on. I doubt that she is faking it.
I agree that the mare in question probably did or does have something else rather than faking going on BUT...
Many years ago a friend was looking for a tb, she needed a hunter. I was going to a sale and she said if I saw something good buy it for her (she was going to pay me back of course.)
A gorgeous tb came through the sale, a seven year old very dark bay. He moved beautiful when trotted out but oh my....he had an osselet! Pretty large in fact. I kept watching and he seemed sound sound sound.
I called my friend and told her about him and she said if I liked him buy him so I did.
The next morning, early, she was out to meet her new horse. She just drooled when she saw him. We saddled him up and she took him in our large ring. LAME, DEAD LAME! My heart just sank. I figured he'd been drugged with pain killer.
We turned him out and off he went bucking, whirling and having a high old time. Didn't see a misstep..not one.
Next day saddled up the seemingly sound gelding and lame again. Turned him out and WHOOPEE!
I called my friend and told her to come out and what I suspected. Told her we were going to ride this horse but I sure was going to be sick if there was really anything wrong with him.
We rode and he limped. We rode longer and he stopped limping on the leg with the osselet. He limped on the other leg. We rode and he stopped limping.
He never limped again and she had him for years and years. They hunted and they evented. He was a good one.
What I THINK happened is that he was smart enough to connect that when he was lame people didn't ride so.....
Only trouble is he almost smarted himself into a dog food can. At that time
Hills had canned horse meat for dogs and there were meat buyers at the sale that day.