I've used live clams and mussels routinely to cycle my tank and to feed butterflies. Good think about live is that they don't rot while the butterflies eat them, until they die and the shell pops open, which could be weeks later, at that time everything else eats them up.
There should be no danger of nutrient blooms, and if there's no surplus nutrients being released, there would not be any bacterial/ phytoplankton bloom either.
But yes, if you tossed in a rotting clam, you could be introducing random, unpredictable pathogens.
And yes, the shellfish industry is required to flush heavy metals and other bio accumulating chemicals out of the shellfish before they are ready for sale, meaning once they are harvested from the wild (or farm), they need to be kept in clean water for x amount of time (could be months) before they are safe for consumption.
