We had our main floor and 4 showers tiled by a contractor who was recommended by the tile supplier. Within two years of the tile completion, the shower tile loosed and lifted and water leaked under the tile floor and the floor tile broke. A regional representative of the water proofing system used by the tile installer inspected the water proofing system and confirmed that the installer had not installed the system properly as the installer left gaps and included improper materials in the showers.
Probably, but it is hard to be sure. It depends on the contract and what warranties are made. Hypothetically speaking, you might ask a tile contractor to install a Kerdi liner and tile over it, but that system may be out of his experience zone. If the contract specifies how the liner is to be installed (perhaps simply via reference to the technical standard), then not following that standard would be a breach of contract. Otherwise, the question would be what constitutes minimum standards in the profession. You can always find someone who will testify "We do that all the time", and you can always find someone who will testify "I would never do that", so it comes down to the question of what a reasonably prudent contractor would do. To the extent that you made the contractor aware of the special requirements of the system, his failure to install according to manufacturer's specification would be negligent. To the extent that an ordinary contractor cannot reasonably be expected to know of the special installation requirements for the system, they may be able to escape liability.