I think this thread question requires more clarity on just what you mean by the "backbone".
From a worker in a company perspective, what is the difference between the "heart and soul" and the "backbone"?
To me the backbones of ECW were Heyman, Styles, Dreamer, and to some degree Gabe Sapolsky.
Throughout the time the compnay existed, those guys wore every hat that needed to be worn. Aside from being the "booker", the "lead commentator", a "wrestler", or the "top assisstant", these four guys also shared in everything else that was needed to keep the company floating like marketing, promoting, booking venues, PR, etc.
Perhaps one of the things that made ECW great was the family atmosphere, and how "all-in" many in the lockerroom were. Guys would go far beyond their normally expected duties to try and keep the company afloat.
As for the others...
Shane Douglas was the first face of the in-ring product, but he was also a notorious lockerroom cancer that ECW didn't need.
Sabu, Sandman and Taz were the other unique longstanding guys along with Dreamer who took up spots wherever needed across the card and remained over with the fanbase for basically the entire duration of the company.
And others like Raven, Whipwreck, Terry Funk, Beula, Axl Rotten, Mahoney, James Mitchell, Bill Alfonso, RVD, Jerry Lynn etc. along with any number of "Dudleyz" also played many important roles for the company for a long period of time.
A special mention should also be made of the elite workers who came through the company during the '95/'96 time range, when ECW was blessed with some of the industry's greatest performers ever if for only a short time frame. Many were just looking for a place to get their feet wet on a bigger stage, a stepping stone to a larger company, or any other number of reasons. But no matter why they were there, their presence helped to truly legitimize the company in its early years. Those names included: Benoit, Guerrero, Malenko, Jericho, Mysterio, Pillman, Foley, and Austin.
So again, without clear terms on what you're looking for, its hard to narrow the field. ECW was always in many ways a true "group effort".