Hey Kevvie!! All okay?
It's not all black and white, but one could write a book about what you're asking. In a very tiny nuttshell and not in any way trying to give an accurate historic analyses:
Cream inventors of heavy rock? Hmmm, no, don't think so. That were Led Zepp, Deep Purple, Jimi Hendrix and Black Sabbath (the latter was inventor of Heavy Metal).
True that Cream was a small first step, but Zepp and Purple realy did the trick. Cream merely made the bridge between the blues and what later became known as (hard)rock. Still Cream was much more blues then rock!
Clapton's influence, IMHO, does go a bit further in guitarplaying. Just like Hendrix (although his influence was much much bigger on hard rock) he was one of the founders of the electric guitar sound we know today. Hendrix was basically the first to distort his guitar and use feedback as music, the founder of the 'heavy' guitar playing. While Clapton, utilised that technique for the blues. Maybe you could say he was one of the first major artists in blues to do the smooth electric solo-guitar stuff, whilst in early days pretty much all blues music was based on acoustic guitars...
My view, some people will probably kill me for saying this... ;-)