I'm not sure one needs to prove that its existence will be defined by suffering that overwhelms all the benefits to its existence - merely that a sufficient amount of suffering is likely, and the benefits are equally obtainable from a dog without these problems.
It is unethical to bring into existence a dog that will suffer pain every day of its shortened life, even if it is a happy dog and brings joy to ita owners, if the alternative is to bring one into existence with a slightly less deformed head that does not suffer so.