yes
yes
No it would die immediately as they are salt water creatures.
Sand dollars will last years. I have one that is probably 10 years old.
Sand dollars have these cilia or hair-like structures underneath them that are used like legs. They are used for locomotion and if they are not moving (especially for a long period of time), they are probably dead by then.
Sand dollars eat tiny particles of food that float in the water, such as plankton.It feeds off the bottom of the sea on plankton.
Sand dollars survive by filter water through their exoskeletons. The live in shallow coastal waters in many different parts of the world.
Sand dollars feed on organic matters floating freely in water. Sometimes they even catch small larvea of other fishes.
Sand dollars are born from eggs. The female sand dollar releases eggs into the water, where they are fertilized by males. The fertilized eggs develop into larvae that float in the ocean before eventually settling to the seabed and metamorphosing into adult sand dollars.
Sand dollars usually are in the water, yes, but they are found on the shore because they are brought in by the tide. They aren't acctually living, they are like a type of shell, but only flat.
Sand dollars do not mate. They spawn, i.e they shed their sperm/eggs in the water in groups. The spawning happens between August-November.
Sand dollars reproduce by releasing eggs and sperm into the water. The fertilized eggs develop into larvae that eventually settle on the ocean floor and grow into adult sand dollars. So, technically, sand dollars do not lay eggs externally like birds or reptiles.
Assuming you mean the skeleton of a sand dollar after it's dead, no it's not illegal. You can collect as many as you want. I don't think there is a restriction on keeping live sand dollars in salt water aquariums, but they are a shrinking population.