No matter how you tackle such idea, the premise speaks of some tremendous research you'll have to go into so that your story sounds at least distantly reasonable.
Maybe you should connect the element to the stabilization of the wormholes. That way the FTL or whatever you want to call it is connected to the material. I agree with the whole point of view that it can't be an element and can't have the "ium" ending. That is unless the people describing this element in the story don't know what they are talking about which is a perfectly reasonable way to go. Make the name used in the book slang and the information known about the element below par and it gets rid of a lot of the room for inaccurate science.