Sprains that you don't see a doctor for don't count as a real medical issue. And a hiking boot isn't prescribed by a doctor. You just buy them at the store. Therefore, there is no cause to treat it as different from any other shoe. It has to come off.

I've had very bad luck identifying painful areas to TSOs. Whenever I've done it, they've pressed extra hard on the areas I identified. I assume the SOP says to ask where you're sensitive, so that if you are hiding something, you probably try and get that area excluded from screening and they should look extra hard there.

I forgot to take my liquid bag out for that screening. They didn't catch it. However, if I'd been carrying my shampoo in my pockets, in my underwear, or stuffed into a sock, the TSA would have found my shampoo.