I think that you can't connect two body parts to each other (A to B and B to A) I do think that you can do a three way thing. (A to B, B to C, and C to A)
The neck is connected to the head because the head in the game is your head, neck, hair, eyes, and all that is above your shoulders. This means that the neck is actually a part of the head, and is so attached.
Also, this means that if you hit someone's head, there is a chance to hit their neck. There is also a chance to hit the neck when you hit the upper body, but because the connection goes from the upper body to the head, and then from the head to the neck, it means that there is a smaller chance of hurting the neck from the upper body. (Sort of like rolling three dice one at a time, but you can only roll the second die, the head, if you roll a one on the first die, and you can only roll the third die, the neck, if you get a one on the second die.)
If you connect the neck to the upper body, and then the head to the neck, it will reverse the places, with it working as above except switch the places of the head and neck.
Nervous body parts are not required, but the [THOUGHT] tag is unless a creature has the [NOTHOUGHT] tag. All that the nervous body part does is set it so that that body part causes extreme pain/paralyzation when it is damaged, and it prevents that body part from healing damage.
If you remove it from dwarves, yes it will allow your soldiers to keep training, but it will basically make the spine part worthless in acting as a spine should. With the [NERVOUS] tag removed, a dwarf will suffer no ill effects at all from a broken spine other than the effects that they would suffer from any broken bone.