will a mortar and pestle be sufficient to grind fish bones?
Small fish like sardines, pilchards, etc., yes you should be able to pestle these down with no problem at all (although they tend to break down naturally when you mix them with other ingredients.  When I eat sardines and pilchards, the bones are hardly noticeable.

Larger fish would be another matter though.

As with any unknowns, waste a little to learn a lot.  Just try mashing up a few fish and see what the state of the bones are afterwards.