I've got two problems:
1. I saved the single dwarven corpse from my two black tower attacks for a future use in a silk farm. Luck had it that I caught a GCS before I even knew it was there, so I built a silk farm, and my last step was to pit the undead for use as a target by the spider (undead are hostile to friends and foe alike, so it doesn't matter if the spider loses his training). However, the dwarven swordsman corpse doesn't appear in the pitting menu! I've successfully pitted all the elves, goblins, and humans, but the single dwarf cannot be pitted it seems!
2. My third undead force refuses to attack. For some reason they ignore my trade depot entrance (they passed 6 tiles away), and are drifting extremely slowly in random but sort of clockwise direction (10 -> 12 o'clock in 6 months). The only thing I can think of that differs between the forces sieges is that the third one appeared further north on the map. My orchard wall directly borders the flat volcano to on the north side, requiring a slight detour to reach the main entrance, plus my addition of a fourth level on the orchard walls (they were 3 tile high the last time). Well, trees have regrown as well, but I doubt that has anything to do with it.
My first attack refused to move as well, but I got it going by restoring to an earlier save that caused the force to spawn at a different place on the edge of the map which allowed them to slowly flow past my wall. At that time I had a 1 tile high wall along the edge, with open gates, and the blocked line of sight somehow caused the force to fail to figure out where to move. I spent the first year building the wall, and the second one tearing it down...
If I could somehow figure out why the buggers lose all concept of where to go, it would certainly help, since I've encountered it many times, including with titans. When I finally get rid if the enemies my first task will be to provide a ledge over the edge of the volcano for attackers to approach on (the longer term plan is to completely cover the top with a combination of floors and bridges).
If I can solve 2, 1 can easily be worked around.
More info:
Went back the save where the force just has entered the map. Removing the orchard wall's top level does nothing. Sacrificing civilians I've been able to get them to enter my trade depot entrance, but they just walk out again, where previous siegers have eagerly filed in, even when civilians are some 10 tiles away, 4 levels down. They may chase civilians, but then leave again. Building a bridge to allow a shortcut over the volcano does nothing (but the builder insisted on building the bridge from the remote side, despite the path being longer, and despite my attempts to increase the movement cost on that side. He LEFT by crossing the bridge...).
And a final report:
The "siegers" left after a year of drifting back end forth. I caught a cave ogre to act as silk farm bait, but my trained GCS refuses to fire at it. Well, I guess the undead will be back eventually.