I admit I didn't read through this entire thread. 28 pages is a bit much to go through. My point may have been made already.
Rather than commenting whether or not horse slaughter is right/wrong, I would like to point to the cause and a solution.
Unlike cows, chickens or pigs, horses in the US are not raised to be slaughtered. Horse slaughter is necessary because there are too many horses. If you don't like horse slaughter, we need to stop breeding them.
There are too many people that breed a horse simply because they "want a baby horse." I even know of a horse rescue that bred a rescue horse to her own horse! If a rescue doesn't understand the problem with over-population you know things are out of hand.
The same thing happens with dogs and cats, which is why so many meet a sad end at the local shelter.
I do think horse slaughter is a necessary evil. I also think much of it could be avoided by avoiding irresponsible breeding. People and groups opposed to slaughterhouses should be as active in going after the breeder who created the problem in the first place. That solves the problem be eliminating the cause.
Once you have too may animals and not enough people to care for them you have only two choices - put them out of their misery or let them die by starvation.