Dr. Matthew Israel was at Harvard in the early 60's and everyone else you have mentioned were there during the 60's. Israel and Davidson opened practices and eventually programs within New England. A member here spoke of being placed at JRC after their stint at Elan. So right there we know Davidson and Israel knew one another. How or why is another thing.
I don't think that you can
definitively conclude that Davidson and Israel knew one another based on those few facts alone. They
might have known one another, but you really can't say that for sure.
Moreover, from what I understand (which may well be incorrect), Davidson was initially focused primarily on
addiction treatment modalities, whereas Israel was constructing a Brave New Skinnerian World for
chronic self-destructors. The kids in the Behavioral Research Institute (JRC's original name), would have been
unlikely, back then, to have many addiction issues save possibly what Big Pharma had foisted upon them.
It's possible they may have overlapped
vis a vis the use of therapeutic communities in treatment, but, here again, there are differences with regard to the target population which may or may not be significant or relevant.