i don't know what you are doing to prevent skeletons from being raised. all you have to do is make the units you want raisable to can raise does decay. and don't try copy pasting that because that won't work. here is my test map it seems to work just fine for me.
if you are testing it and you think it's not working...here is a tip for you...
after you see the skeleton corpse disappear, then shortly after that you can raise skeletons from that corpse, which you cannot see anymore. heh. i think that's just because the way the model works. the actual corpse disappearing thing is more of part of the death animation thing which it sounds like you were editing, which is not necessary. the longer your death animation, the longer you have to wait for the corpse for you to raise it. technically skeletons don't have corpses so you don't see any corpse but it is there as an invisible corpse until it finishes decaying.
but yeah i'm 99 percent sure your problem is the sixty second death animation time you set. that means you have to wait for the full minute death animation to pass before you can raise the corpse. that and the fact that you seem to think that you have to revive a unit while it is dying. when you revive a hero you cannot begin to revive it until it finishes it's death animation. it's the same thing with the rod of necromancy, and the same thing with the necromancer raise dead ability. you have to wait until the unit finishes dying before you can raise it. in the case of a skeleton, the skeleton has to disappear before you can revive it. because it's death animation finishes at that point. it looks a little funny having necromancer cast raise dead on the ground itself...
you could try adding doodad of corpses trigger for visual. i think they did something like this in undead missions in TFT in single player if i am not mistaken. this will make the whole effect of what you're trying to do seem more realistic. i recommend this. anyway to do what you want to do like i said all you have to do is edit that one entry for the unit you need to raise. i'm not sure if this is the case with mechanical...we can test that too if need be...just go to the siege engine and do the same thing i did with the skeletons. oh and add a few raiders so you can kill the tank faster lol.