The information you include about SPD´s iwill mislead the opinion of
engineers
because there´re manufacturers that had consider the problem and
include a fuse in front
of each MOV, designed to open the circuit in case of a failure or end of life of the varistor.
but it´s not any fuse, because it goes in the protection module in front of each MOV. On the other hand a limiting fuse for protection will jeopardize the function of the SPD