Are Watchtower statistics really trustworthy?
No. Absolutely not.
I have explored the differing reports of JW membership leading up to and following WW2 and I think that the internal division within the Bible Student organization during the 20s and 30s in Germany had much to do with the discrepancy in numbers that you will find from differing sources.
There was a major split in the WTS in Germany in 1925 - Rutherford made a trip to that country and gave the boot to Conrad Binkele, who was in charge of the German WTS, resulting in many of the Bible Students following Binkle, rather than Rutherford. In Germany, even after the name change in 1931 to Jehovah's witnesses, the name of 'Bible Student' was used to encompass all of the Bible Students - both those who stayed loyal to Joe's Army and those who followed the 'original' Bible Student group, led by Conrad Binkele who founded the Free Bible Students in Germany in 1928.
I believe that many of the discrepancies in membership numbers in Germany, both in and out of the concentration camps, has much to do with that split not being recognized by outsiders to the WTS, and that the WTS has exploited that misunderstanding. By including all of the Bible Students in the 'JW' numbers, the Watchtower Society has been able to inflate their numbers and massage them whenever appropriate.
The separation of 'Bible Student' from 'Jehovah's witness' was based upon a doctrinal shift - those who stayed with the Bible Students held onto the Zionist doctrine started by Russell, and those who followed Joe adopted the replacement theology. In effect, there were two distinct groups of Bible Students - one doctrine (Zionist theology) put those who followed it in direct opposition to Nazi ideology. And the other doctrine - Joe's replacement Theology - was more compatible to the Nazi position.
It took Rutherford until 1932(?) to make that about face in doctrine, just in time to make the plea he did to Hitler in 1933.
The WW2 concentration camps, in effect, became an ideal place for the 'sifting' within the German WTS organization to take place, and that could be the explanation for why there are differing accounts of how the JWs were treated in the camps - those who followed the 'old' Zionist theology (Bible Students) were treated far more harshly than those who adopted the new replacement theology (JWs).
The Watchtower Society has embraced all the WW2 purple triangles as their own - when in reality, many of those purple triangles were not part of the Watchtower Society at the time.