
Ultimately and MORALLY it should always be the breeder's responsibility. Rescue shouldn't EVER come into it. A responsible breeder should always take a dog they have bred back if there is a problem and the owner feels unable to keep the dog -for any reason at all and at any age. But LEGALLY, an unlicensed breeder (i.e. the great majority other than commercial breeders) has no legal responsibilities and it is a case of caveat emptor -buyer beware. In law, it is the buyer who has to ensure they buy a pup they are happy with as there is no come back on the breeder. If the breeder however is licensed, the breeder has more legal obligations, similar to any other shop selling a product.
A breed known to be friendly and happy with strangers should be happy even when they have pups. A breed known to guard, such as my own main breed, I would accept a bitch to be protective of her pups and not want visitors but ONLY up to the age of 3 weeks or so, after that there should be no need to feel that protective. I have one bitch that won't actively welcome visitors into the house, that is part of how the breed is -in some ways they ones that throw themselves at visitors with wagging tails are more untypical. But take it slowly and give her a few treats and she comes round, every single time.
Anyone can end up with a bad litter, no matter what we do it can happen, and indeed I had a problem litter 5 years ago. Some of the pups were fantastic, some had major behavioural problems, and it certainly was through no fault of the owners. When something like that happens the breeder should stand by their puppy buyers with help and advice of whatever form is suitable, and I hope this is what I have done with mine. I also ensured NONE of the dogs from that litter were ever bred on from (including the ones with GOOD temperament and no problems at all).