So are you questioning Tel Aviv University's Leonid Smilovitsky's interp of Guggenheimer & Guggenheimer's Jewish Family Names and Their Origins or just the way he wrote it? Are you questioning my interp of Smilovitsky's statements because he wrote Katz - Katznelson? Or are you questioning why someone would not accept the original Oxford Names Companion?
I was interested in the architecure of the name and I believe the OP was as well. That is how I saw the repeated question. Some people don't equate knowing the name's "place of birth" as knowing the meaning of a word (or the words used to form the name of origin). Since the OP was more concerned with Katz + Nelson, I thought Simlovtinksy on point in that it was the -son that was added at a later time. That is to say that it's not about being Nel's son + cat. This is why I quoted him as an expert. I have no problem admitting my supposition on -nel- added as something I don't know about or as a better way to connect katz with son as being incorrect as I am not nor ever said I am an expert on Jewish names. I used the information I found and if I made too great a leap, then I apologize.
CORRECTION: Katznelson means son of Katznel, which, according to Oxford Names Companion and others, is an abbreviated version of Katzenellenbogen. Thus we see Katznelson is a locative name. Although folklore says the origin of Katzenellenbogen is associated with a "cat's elbow," I shall not suppose further on the actual meaning of this word or any part thereof.
~Infamous? I think I like that. I'll take two orders of infamous, please.