I am of course no expert, but...
It looks like the page from the 1936 yearbook is showing the count of Memorial partakers & attenders for the US branch only.
Of the 27,006 partakers, 26,826 are under the column for the "United States". There is another column for "outlying countries", but there are only 300 listed as "attending" in that column.
Again, I don't have info handy, but weren't there many thousands of JWs in Canada, England, Germany, etc. in the 1930's?
It also sounds like the 1996 Questions from Readers is making reference to the total world report (63,146).
I don't know what they mean in the '36 yearbook by "outlynig countries"...perhpas they mean Hawaii & Alaska, which were US territories but not states at the time? Or might it mean persons "assigned" to the US branch, but serving as missionaries elsewhere?
Of course, the issue with the more-or-less constant count of 8500 partakers over the past 15 years remains. Whether you start from 27,000 or 52,000 in 1935, it is actuarially impossible that 8500 of them survived 70+ years...either these guys are getting their body parts bionically replaced so that they never die, or the majority of the partakers between the 30's & the 90's ended up being unfaithful & had to be "replaced". Neither possibility is something the Watchtower likes to comment on.