1.The tail is trimmed because a dirty or offcolor tail can cast those offcolors onto the chinchilla's body, causing the chinchilla to look casty. Generally, on the tail, the tip of the fur is lighter and can get stained easily. The tail fur might also not be as bright as one would like. By cutting this fur, you will prevent those offcolors from being cast onto the chinchilla.
2. I don't know if one would need to trim the tail of a **** ebony since the tail would be solid black. I suppose if it was exceptionally dirty or if the chinchilla held his tail so that it mussed up the fur on his back, it would be a good idea.
3. It looks almost the same as a regular chinchilla's tail, just with shorter fur... and some do a blunt cut on the tip of the tail fur as well. I trim the tails of all my show chins but I do try to blend it in so that the trim looks natural. This is just my personal preference though.