As already acknowledged, JWs preached before, during and after this period. So the connection between tax status, preaching, and 1975 looks tenuous to non-existent. Have you missed out some evidence for a connection?
Besides, if you are accused of not being a real religion, then setting a date for Armageddon to prove that you are, is a pretty bizarre response, given that society in general doesn't tend to view apocalyptic date-setting favourably.
The evidence would tend to suggest that JWs promoted 1975 because many of them really believed it, including the Vice President. It's fair to argue the leadership were also pleased about the boost in membership and activity and that might have been part of a calculation for some.
You argue the real reason for the blood situation is money, and that money also prompted the 1975 prophecy. Is it your general contention that financial considerations are behind most JW doctrinal history?