Lin, I don't thing there is a "proper" spelling for the common name. I believe it began as Crepe Myrtle (still used by many gardeners) but over the years has also morphed into Crape Myrtle (the spelling for the way it's pronounced in the US). Both are used and "correct" (at least as far as any common name can be correct). Have never seen it listed as Crapemyrtle except for your citation from ITIS. I think 99% of gardeners would look for it as 2 words.
Searching on the plural for the parent plant is rather odd. It really should be just the singular. One doesn't look up Peonies; it's just Peony on the search.
Those of us who dislike this overused, suckering, aphid-infested plant which is almost impossible to get rid of call it Crap Myrtle (not a common "common name")
