Until I read Neil's book I was under the impression that Graham made the one visit to the US and that was in late '72. However, as you know, he said in the book that he 'first' went to the USA in 1971, implying there were other trips after that. When he talks of the big find in Miami, he is not clear about exactly when that was. Was that '71, or later?
I suspect later because, as you say, if those records had been discovered in '71 then playlists in '72 would have reflected this, and they don't-until the autumn. From that point onwards many of the listed records were being played at the Cat's and the Torch (until its closure)
The only evidence I have for the original Miami find being in late '72 is a memory of a buzz of excitement around the DJ booth and the change in the playlist that occurred at the same time. I don't recall that happening anytime prior to that, or afterwards to the same degree. Ian Levine is definitive that his big haul was in the summer of '73, which fits that time frame.
I take your point about the cost of flights at that time, and also take into account Neil Rushton's and Ian Dewhirst's assertions that Graham was going to the States from 1970 onwards. All in all, much of the evidence precludes a100% definitive conclusion.
I suppose none of it really matters. All we can say is that it was great to be going to venues in that era and hearing such great sounds, no matter who found them.