I did more tests and think I've figured out the problem with the arm. First off, incase anyone doesn't know, when the player equips hand armor, SoM replaces the swing arm with 3 new segments that are part of the hand armor MDO model.
The arm does need 4 bones (upper arm, forearm, hand, weapon), but the bones have to be in a specific 'order' in the MDL. Mick's x2mdl might have a bug in that it 'sorts' the bones in reverse order from what SoM expects. That makes the weapon move with the 'upper arm' segment, and all the 'hand armor' segments are reversed too. Mick could probably sort it out without much effort, if anyone wants to go over it with him on his site- but I'm too busy right now to get into that.
You can technically make a skeleton with an extremely convoluted structure that will make the arm turn out correct ingame, but it would be almost impossible to animate. Example is attached- try it with and without hand armor equipped.
Also- SoM expects the arm to start in a downward position on the first frame of each swing animation- like it is holding Verdite's 'sword alignment' mode. And then move up for the normal swing animation. This is because the weapon and hand-armor-segments move with the arm's parts so they won't align properly unless you start the arm in the expected position.
Lastly, x2mdl has a bug that sometimes instantly flips part of a model. Mick said it was fixed, but it's not. You'll see the effect in the attached sample arm MDL. We might be able to work around this problem by changing FPS if he doesn't fix x2mdl.
Sorry for the long, probably confusing post. 
The arm does need 4 bones (upper arm, forearm, hand, weapon), but the bones have to be in a specific 'order' in the MDL. Mick's x2mdl might have a bug in that it 'sorts' the bones in reverse order from what SoM expects. That makes the weapon move with the 'upper arm' segment, and all the 'hand armor' segments are reversed too. Mick could probably sort it out without much effort, if anyone wants to go over it with him on his site- but I'm too busy right now to get into that.
You can technically make a skeleton with an extremely convoluted structure that will make the arm turn out correct ingame, but it would be almost impossible to animate. Example is attached- try it with and without hand armor equipped.
Also- SoM expects the arm to start in a downward position on the first frame of each swing animation- like it is holding Verdite's 'sword alignment' mode. And then move up for the normal swing animation. This is because the weapon and hand-armor-segments move with the arm's parts so they won't align properly unless you start the arm in the expected position.
Lastly, x2mdl has a bug that sometimes instantly flips part of a model. Mick said it was fixed, but it's not. You'll see the effect in the attached sample arm MDL. We might be able to work around this problem by changing FPS if he doesn't fix x2mdl.
Sorry for the long, probably confusing post.