From bottom to top:
If in a stockpile, food won't rot. Vermin might decrease the stacksize of the food though, so it's a good idea to have some cats roaming around.
I managed to mod that once, yes. I think the issue was the [INTELLIGENT] tag; until I removed that from the humans and goblins, the dwarves refused to butcher their corpses, even after I had set the various ethic tags of the dwarves in entity_default in a way that should theoretically allow them to do these things. There are some ethics that are UNTHINKABLE by default. These must be changed into ACCEPTABLE.
Butchery:
1. You need some animals for this. Let's assume that you have wasted some points on the embark on a breeding pair of doggies for the purpose of this explanation. After a year or two, their amount will have multiplied and they start to really annoy you.
2. Install a butcher (b-w-u), a tanner (b-w-n), a leather stockpile (p-l), a leatherworkshop (b-w-e). Have a large stockpile that accepts food, because your butcher produces a LOT of items upon the slaughter of an animal.
3. You need to have a dwarf with the butcher job enabled. Dwarf Therapist helps here. If you don't want to waste the skin, then you also need a dwarf with the tanner job enabled.
4. Two methods now:
- first, the clumsy micromangement option: "v" next to a doggie. "p" for preferences for this dog. "s" for "ready to be slaughtered". Advantage of this: while you are "v"ing the doggie, you can also "z" at its description and only butcher the weak and pathetic ones to breed superdoggies in some generations.
- second, the more general approach: "z" to get the inventory of your fortress. Hit "enter" to select "animals". They should be sorted accordingly to their age. So, if you think that your oldest doggie is about to collapse from old age, just select the top couple of them. Else, select whatever you want to slaughter from the list. Hit "enter" to toggle this animal to be slaughtered.
Dogs are relatively small animals, so maybe they don't explode into as many items as a cow does, but you will notice that your food stockpile is being filled with a lot of "prepared dog brain", "prepared dog kidney" and similar things. A good way to reduce this clutter is to convert it into lavish meals in your kitchen. The kitchen is also necessary to render the dog's fat into something usable.