A couple other semi-related things:

First, the human siegers flee from my dwarves, although they will use bows if they have them.  Looks like they're using the civilian AI that hunters use concerning battle.

Second, cage traps catch the mount, but not the rider.  Seems like they should catch both.

You gotta love that groundhog swimming pool, though XD  I imagine that the mommy pathed into the pool when it was half full, and then it dried up.  Why they didn't try to path out of it when the pool began filling up again (though it never made it all the way before the dry season hit again) is a good question.

Updated to 40d, but the crash still happens, although about a week later than before.

This time the crash seems to be related to the screw pump on the right side of the fortress one level below the surface.  I haven't figured out why yet, but it's crashing during the regular old water pumping routine.  I've disconnected the axle in that wooden siltstone tunnel, and I'm currently running it to see if that prevents the crash.  Of course, I have no idea if that pump is crucial in some sort of dwarf survival/productivity kind of way.  Perhaps I'll figure out before I reach the 10th of Malachite.

Update:  I got pretty far along (14th), and it seems fine.  Still not sure on the root cause.  I guess the pump runs that waterfall pillar circuit.  Perhaps when enough water evaporates something is becoming unstable.  Probably just have to find a typo somewhere.