Eddie.
To answer your question.
Unless the aircraft is controlled by a maintence controller the the person (lame) that signs of each cat is and the lame co-ordinator will be deemed the responsible as we'll as the owner.
Also where in the reqs dose it state a wavier is legal. If the sb is recommend the casa has give a direction that it is now recommend and may be viewed as that. If an Ad states a Sb then it must be done if it's applicable to the aircraft in question.
If the aircraft is under the controll of a maintence controller then they take the reponasabillity for the paper work and the lame takes resopability for the work carried out.