Hi, I just realized that what may have killed your snail & shrimp could have been the urchin. Many sea urchins have venomous spines, if the shrimp or snail came near it, this may have been what killed it. The urchins predators are snails, crabs. If your tanks chemistry checks out, I would NOT do that drastic water change, it may stress out the other fish and create a whole new can of worms for you.
I would do a 20% if needed and let it be. When I first started my tank I bought some clown fish, my favorite and some other fishies. I also bought a brittle star, they are beautiful, the aquarium store owner told me it was OK to put in the tank with my other fish. Yeah! it ate my scooter blenny, one of clownfish. I took the darn thing out of the tank, and took it back. From then on regardless of what the aquarium guy told me I would research on the internet or books any new inhabitants that I wanted to bring home. One of my most special treats while owning salt water aquariums, I had 4, one in every room, my anemone split itself, 2 x's, she was in a 10 gallon tank with a beautiful black clownfish. Okay, I'm starting to miss my critters again. : (