1. I have a ton of xxx skeletons or xxx partial skeletons, yet I can't do bone greaves with them. How to use these skeletons? How to make bone greaves?
2. I keep stockpiling large roach remains? I think they are useless, but will they disappear eventually from the refuse stockpile?
3. What to do with turtle shell?
Ad 1) If there is a butcher's shop within 20 (30?) tiles, and it won't butcher them, you can't use them. You can wait till they rot, hope they turn into a stack of bones, or dump/relocate.
Bone armor is made in craftdwarf's workshop. If you have metal, use metal instead, even copper is much better than bone. You need bone to make bone armor.
Ad 2) You can go to {o}rders {r}efuse ignore vermin remains. They clutter refuse stockpiles, and generate hauling jobs, but rot and disappear eventually. You can also get rid of cats to have no vermin remains.
Ad 3) Nothing. Keep it for strange moods. If you have over 20 you can consider making shell crafts or decorating with it.
On all 3: I build 1 big (up to 30x30) refuse pile outside, set it to not accept bones, hooves, shell, horn, tooth, etc. I surround it with a fer butcher's workshops and tanneries. I also designate a refuse pile inside, that accepts
only bone, hoof, shell, horn, tooth, and put a few craftdwarf's workshops close.
The consensus about use of bone seems to be bone bolts for training marksdwarves.