I'm afraid AnDruidh the history I've heard about the Pai Lum teachings varies quite a bit from yours.
Pai Lum Kung Fu was either a Pai family system or created by Daniel Pai. I know of no way to verify either point of view independently.
As to its course content varying, likely the truth. I've seen several different flavors of Pai Lum, some very closely linked to Chinese systems, and some that appear to be karateka doing kung fu, IMO. I'm quite curious where the statement the original source of Pai Lum was Goju Karate arises. Of course they do have a tension form that is very similar to Goju Tensho, yet also quite different, different breathing patterns, etc.
As to Manuel Agrella's being a source of Pai Lum material I highly doubt that. I know at one time he did live in Conn. and there may have been some interaction between the parties you mention. But I find no similarity.
For years I competed on the circuit against Mr. Agrella's people. Their material was absolutely nothing like the Pai Lum material. Agrella Sifu claimed to be a student at one time of Mas Oyama, in my presence. Then again I found nothing similar to the Chinese Mantis systems present in his student's forms either.
But China is large and I'm sure I haven't seen all there is by any means.
But Agrella Sifu's people were hard, fast and very skilled at what they did. It's just I personally can't reconcile their art with any of the various Chinese systems I've seen.
Also as I heard it, it was Pai who taught the Tracy brothers their self defense techniques.
The entire impact of Pai's teaching is difficult to asess. In the 60's and 70's he was a presence on the East Coast.
But thank you for mentioning Agrella Sifu. I had not thought of him and his art in several decades.
bushi no te isshinryu