Is it right, that the tax involved was property tax in the state of New York? It's not the same as JWs losing tax exemption in general. Any idea about the amount of money involved? If they paid the tax from 1971 to 1974 is there public record?
Yes I read the part of the article that says preaching was key to their argument for being considered a religious community. What is absent however is any evidence that 1975 was promoted with the aim of proving that they are a proselytising community when they were already regarded as such. Instead what we have is a lot of evidence that JWs were pointing to 1975 for decades (at least since the 1940s) and that Fred Franz heavily promoted the idea and was idelogically committed to it.
As dozy says, many recent developments were clearly financially motivated, such as dropping charges for the literature. The evidence for the financial and legal motives behind that change is clear.