In order to have the weapon on the back you need to change the property "SideWeapon" to "BackWeapon" in the "NiStringExtraData" block that has the name "SideWeapon" (how surprising). You also need to change that name to "BackWeapon" as well. So the mesh will have this structure after wards:
0 NiNode LongSword1 BSXFlags BSX2 NiStringExtraData BackWeapon
Unfortunately the one- and twohandedness of weapoons is hardcoded into the game so you are going to have to use the NifScript utility to be able to change the sword from twohanded to onehanded. There's a thread here somewhere, I saw it yesterday so you won't have to search too long to find it. Hope this helps.