Here's how I remember it:
Bob Kasper trained predominately with Charles Nelson for his H2H combat. I believe both he and Cestari both trained with Nelson at the same time. Bob and Charles were great friends as well, Nelson even came to one of the barnyard getaways and worked with everyone, which included a lot of old jujutsu tricks of putting you on your back (or face).
I never knew where Bob got his knife training, but I do know he got elements of it from some of the old Marine Raiders and other WW2 vets he interviewed. Specifically some of the old Styles' methods (based off fencing) such as inquartata, pasatta sotto, snap cuts, etc. He basically "updated" these methods while keeping the essence of the original techniques. Then, as mentioned before, the shanking methods he got from some guys from Rahway State Prison, not to mention Bob was into some of the 2% motorcycle gangs out there, I believe he mentioned knowing a couple of the original Hell's Angels as well. Bob was a heavy banger, that's how he liked to live.
John Kary I believe got his training from Cestari, not sure about Nelson. I believe John was already doing his combatives before Bob showed up with the GHCA.
Bob's and John's unarmed combatives were different, but similar. John used "striking sequences" - which changed often - and self defense scenarios, with a very few jujutsu techniques thrown in, after striking of course. John also taught neck-breaking techniques that melded right into the sequences. Bob had striking sequences as well, but these never changed and were created for muscle-memory and to achieve a flow from one strike to the next. They were similar as all the WW2 strikes were utilized: axe hands, chin jabs, elbow strikes, stomp kicks, face smashes, eye gouges, etc.
I'm not sure if anyone is carrying on the teachings of the GHCA or not. Up in Brick, NJ, Bob's most devout students were John Watson and Joe Kanabrocki. Up in Long Island, NY, John's long-time students were John Martin, Gene Duenes, and Lord help me I can't remember the older guy's name. He's in John's book titled "American Combatives" (as am I

).
I have heard recently that John Kary has passed the reins of American Combatives to Joe Green out of Longview Texas. Joe is a great guy, I used to train him for John when I lived over in East Texas, and apparently Joe stuck with it, and in order for John to do that then Joe is good to go.
My background was predominately with John Kary as I stated before, but I was lucky enough to train with Bob and Kelly as well in their methods.
I did achieve Instructorship status from both AC and the GHCA. I used to train some of John's students when they would fly into NY to train, one was Dr. Thomas Nardi (he used to be a prolific writer for several martial arts magazines). With Kary's blessing, I also opened a satellite American Combatives school back in 1994, but with my crazy ex-wife all of that fell apart before it started. I also trained a group of militia guys in Southern Arkansas for a few months. I don't really teach anyone right now, just some guys from where I work from time to time, nothing serious - just some smash mouth techniques and a little knife. However, like I mentioned, I have taken different elements from all of my WW2 combatives teachers and have created my own curriculum of unarmed, knife, stick, and gun. To me, these are the "best of the best" techniques and delivery methods for those techniqes, at least regarding WW2 Combatives.
Again, with McCann, Kasper, and Kary it was all about attitude. They could probably take a few traditional tai chi techniqes and make them highly effective with this alone.