Tuvok was salt, Neelix was water, and Tuvix was salt water. The Doctor simply found a way to separate the salt water into its original components again. Nothing was created or destroyed that wasn't already there.
Uh, no. Sentient beings are not merely physical components. A sentient entity is a system
-- especially a mind, which is a complex system of neural interactions. When that system is dismantled and its components used to create a new system, that person is therefore dead. You might as well argue that George Washington isn't really dead just because the particles that made up his body were later absorbed by other organisms.
Besides, Tuvix clearly did not have the actual physical mass of two men; ergo, a significant amount of physical matter was lost in his creation.