Don't get the wrong idea, my work is far from perfect. I just found good ways to fake it enough that the illusion of freely switch gripping is created. Under the hood there is a lot of "oh no you don'ts" where if the player tries them it will auto switch back on him. Like trying to use the left hand for example.
Sorry man but there are a lot of situations that you cannot allow the switch grip or you will get very bad issues. Even JUST switching back and forth in succession too quickly will cause combat actor data damage (temporary damage but serious enough to require a quit and reload to correct it).
And even when you succeed at it without "obvious " issues (that the player will see(... any weapon BASE you switch grip means ALL actors will use ALL weapon from that base the same way. It mostly works with the player because the player almost always has a unique weapon with one they have smithed or an enchanted one or even one of the one of a kind weapons in the game or form mods.
Why not just MAKE custom greatsword that looks like the normal greatsword but is made int he CK to be 1 handed, and then SWITCH (unequip the real one then equip the trick one) them via a scripted ability whenever the vampire equips a greatsword? That would be the way I would do it.
That was the way I was going to do it with the player as well but then I realized the player almost always has unique weapons.