1. I have no experience with reading pz3 files and cannot open your scene without your character props and textures. Refering to real life I think that sword should be parented to hand ;)
   As was metioned above you can parent object to object for whole scenes. Parents cannot be changed during animation.

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7. I attempt to save the current pose to the library, as it's one of the ones I want.. At this point, my figure's neck stretches stupidly long, the left arm goes into a straight out pose, and the sheath on the back falls to the floor. What the hell is going on ?

Your pose cannot be saved correct, because some body parts are animated refering to prop. For example transfering X unparented hand you move it along body X ax. Parented hand moves along prop ax - if you rotate prop along Y ax X ax is completly different.
Ref neck behavior

• you could generate some bug. Sometimes, when I calculate cloth dynamic, head of figure explodes or hairs fly away :D It happend in complicated scenes.
• you can set IK for head or neck, when you changed IK chains.
  Open again your scene and check what body parts you checked saving pose.

3. If you want to change parents during scene you can use 2 swords (with dummy props). Check prop/sword properties - "visible" option can be animated. Animate swords together setting 2nd sword unvisible, when you are in desired frame, partent hand to 2nd sword, make unvisible 1st sword and 2nd visible.

Anyway you are trying to make something very complicated. Remember that you can make 2 scenes and later join avi files in 1 movie.