The best way to control/reduce those chemicals is to use activated carbon periodically. I don't run AC all the time because it takes out too much of the good stuff, but for many chemical compounds, that we sometimes call toxins, it is the only thing that works. Skimmers don't do it.
Personally, I've found that some Sarcophyton do well in one tank and lousy in another tank. Leather coral aren't as much bothered by each other as each variety is bothered to different degrees by other chemicals produced by other soft and hard coral in the same tank. Then I've had leather coral that didn't do well in anyones tank.
In other words, after reading your post, I must admit, I don't see it the same as you do. What I see is a Leather that wasn't doing well and may not have done well in any tank regardless of it's tankmates. While it was dieing, which took months, it polluted the tank with it's foul "smell". When the coral was finally gone and the "smell" decomposed, the neighborhood could finally come out and play.