I think you are spot on with your thoughts.
It is up to the RP to ensure that persons nominated to assist in evacuations are competent. It is up to the RP to ensure that staff are given appropriate training and that the procedures given are adequate. Now if the RP doesn't bother supplying training, and simply informs someone that they are the marshall, and they have to fight a fire etc then the RP has failed in his legal duty, not the 'marshall'. If the RP supplies the correct training, and gives rise to suitable procedures, and the marshall decided to completely go against the procedures, or simply doesn't perform the duties expected of him/her then there is a potential offence under article 32(10) of failing to comply with article 23. (As you point out)
There is also the more general duty of care to think about.