Unforunately some anemones will move, and not always to a place that seems best.

But usually, if they die within a week or two, it has more to do with water conditions than with lighting levels. In the past, "reef"keepers kept anemones under T-12 lighting, but this only supported the creature for about a year or two. Seems like a long time, but remeber that anemones can live potentially
forever in the wild (like a tree, sea anemones do not have any body parts that will 'expire' after a number of years, such as a heart). "Sticky" anemones like Seba's Anemone and Adhesive Anemones usually do not move as much as Long Tentacle Anemones,
Condylactis, et cetera.
You are also right in that Clownfish do not require an anemone host, and vice versa. However, they both tend to do a little better in each other's presence. Clownfish potentially can host in
Condylactis as well.
Convict Tangs are excellent schoolers, and use their numbers to overwhelm other grazers. I was actually a mere two feet away from an occurance like this; it was in Hawaii, and a large shoal of Convicts (or
Manini) "attacked" several
Naso Tangs. Even though the
Naso's have two pairs of enormous tail spines, the Convicts drove them away with numbers. Apparently the mere sight of a Convict Tang is enough to get many species of grazers fired up.
-Lynden