Whoo boy. So many opinions, so little time, so little desire to have everyone hate me.
Let's go with #1: I am entirely in favor of horse slaughter.
I am not a vegetarian. I eat meat, primarily poultry. I try to make good decisions about what I eat and when I eat, but as much as I love animals, I have no reservations about how delicious they are.
If we slaughter animals for meat, there is no reason we should not slaughter horses as well.
The overpopulation problem is huge. There are many horses out there who will live desperately miserable lives through neglect or mistreatment. They will never be trained up to a useful career. There are many terrible human beings out there who would rather let them starve in a field or abuse them rather than give them a humane end of life.
That said, I have reservations about the way horse slaughter is done in North America. Captive bolt is a terrible way to kill horse, who are not the placid animals that cattle are. Rounding mustangs up in order to slaughter them is really atrocious wildlife management. And finally, if horses are going to become part of the food supply there need to be strategies for regulating the medication & supplements that we use in them.
I will never personally eat horse, much the same as I choose not to eat lamb and would choose not to eat dog if it were presented to me. That's my personal choice. But I don't think it should be ruled out entirely for sentimental reasons.