I can't see how it can be diagnosed as rage at nineteen weeks.
Clearly, your MIL is not being firm and consistent in bringing up her dog. I don't profess to be an expert, but I have raised three Cocker Spaniels - one black, two golden -and I have never experienced that sort of behaviour. You cannot allow puppies to have their own way, otherwise you end up with an undisciplined dog, that is no fun to own, and possibly can't be trusted. A firm "NO" must mean precisely that, and the puppy must have clear boundaries of acceptable behaviour. Positive reinforcement is essential, and smacking or hitting is out of the question.
I don't think my Charley would win any awards for obedience - he's a typical Cocker, and selectively deaf when we are out and he's off the lead - but at home he is well behaved and does as he is told: he is loving, and in turn is loved.