From what I know, my family started using "Kantzow" most consistently after the 1850's-60's; it was only by luck that I discovered the true surname was Cantzoff. The more successful Kantzows who were living in Stockholm when Sven was living there seem to have no relation to him at all (though I tried to find any links, in Stockholm and in Vaasa, Finland).
His mystery is odd and frustrating. With diligence, my own stubbornness and greatly appreciated suggestions from others, I have been able to find out a great deal about my other Swedish family lines. But Sven seem immune to my most earnest persistence.
I do have a query in to the Stockholm archives re: his membership in the tailors' guild; if those records exist, perhaps I will find out more. If that comes up empty -- ?
Thanks again for taking an interest; I do appreciate it!