From what I understand about Rutherford and his need to control everything I can't see him using anyone as a ghost writer. I CAN easily say that I can see Rutherford up til the wee hours slaving away at his vitriolic rants at the Catholic Church, religion in general and politics
If there are writing similarities wouldn't it be possible for Franz to have copied the style set up by Rutherford.
I suspect there are many instances of the same style of writing to have been copied over time.