They don't take on different forms depending on the wielder, they are just reforged from time to time. Except Amoraccius, which has been the same since the days of the original Merlin.
Inherited warden swords are actually a neat plot hook. A warden in my pbp campaign has inherited the sword of his father along with the responsibilities as a warden. I think the father-son connection is enough to make the magic work.
In the case of a mortal, I'd be bit reluctant as well, but I could see a way it could work rather nicely. Give him the sword, make it a weapon:3 and that's it. It is pretty much like any mundane sword, except maybe for a little more oomph. However, you also give him the option to spend a fate point to activate one of the warden swords special abilities (weapon:6 or counterspell:6) at one time. After that he has to make a choice. Either that action was enough to spark his own dormant magic, or the weapon just had one charge left and is now fully depleted and will never be anything but a very good sword. The loss of refresh would then come from inside the story instead of simply from the fact that he is having the sword, which would make the decision much more meaningful.