Hey Audsley.............
I'm sure that no clams or oysters are kept frozen for later use in RP. The standard when eating so called "fresh" clams and oysters is for them to be alive. That means the animal still has the strength to keep its two shell halves clamped tight.
If you do buy "fresh" in RP be sure to inspect every single one, as those animals may have been sitting there for days. Eating a dead one can be as hazardous as eating a live one infected with Red Tide. Most intertidal clams have evolved to stay alive through several hours of exposure during low tides. Many of them can survive a day or more if kept cool, shaded and wet. The large white clams that you see in the fish market are deep water types that cannot survive being out of the water for very long.
I've seen them sitting in tubs of water there as if that will keep them happy..wrong! All that does is kill them even faster as they will deplete the oxygen in the water in short order. I've told those guys many times that a simple aquarium aerator will keep them happy and guess what? Some of them are actually doing it now.
As for your safety when eating them after a Red Tide event, in my opinion I'd feel safe after a few days as the toxin is concentrated in the gut of those animals and is rapidly passed through in a matter of hours. As for buying them frozen, I would never do it since you don't have a clue as to the condition of the animal when it was harvested. I won't even eat those so-called "fresh" but dead oysters that are sold in a jar at our local super markets.
JJ