You don't need academic knowledge of anatomy if you grow up in a culture (that of medieval peasants) where hunting, cleaning, and cooking animals was likely common. Much can be inferred from analogy, in the way the non-human dissections were (and still are) used for training surgeons.
Probably. Clearly if you happen to have a butcher at hand to get you the liver. Not sure if nowadays you could just bring a random dig to a butcher and ket him extract the liver for you - might be hygienic standards preventing to do it. Earlier might be the belief that dogs are dirty etc. but i don’t know...
Humans are one of the only animals that don’t eat poop. The benefits of poo exposure is an increased diversity of gut bacteria and biome, which we are just starting to understand the benefits of.
Given the hygiene standards in the middle ages, humans were very much exposed to poop of all kind of sources, so I doubt that additional exposure would be of any benefit.
It is not that dumb.
Eating camel dung for example helps with serious stomach infections, its old and well known Bedouin trick (tested by Wehrmacht soldiers in Africa during WWII).
Apart from parasites and bacteria shit could be beneficial in theory - as its full of minerals and could help to regenerate skin etc.
Same reason why most 'magic potions' include hard to achieve steps (steal church parchments, estract the liver of a dog, etc.).