I'm with Rick on the statement of facts. What we call bluegrass was heard for the very first time that celebrated Grand Ole Opry night in 1945. Also, that Bill didn't call it bluegrass music until the rest of the world had already turned his band's name into a generic term.
Still, to say that Bill is the single "inventor" of bluegrass is something like saying Jelly Roll Morton is the single "inventor" of jazz (a claim Norton always defended). Clearly there were many influences in both country, pop and jazz music of the day that found their way into what we call bluegrass. Monroe is generally referred to as the father of bluegrass music not it's "inventor."
I made a series of MP3 files to try to demonstrate the development of Monroe's music over time. They included The Monroe Brothers, the "pre-bluegrass" Blue Grass Boys, the original "bluegrass" band and a track from a recording made in the early seventies. I had them posted on a web site but I've changed ISPs so I'm sure the're no longer there. I'll try to find somewhere to post them and let everyone know. It's an interesting progression.