You should have nprinting installed on a seperate sever to sense. part of the setup involves the import of a certificate from the sense server onto the nprinting server so wonder if you will get issues. also nprinting as a tool is a hungry beast and it will eat all your available cores and ram to generate the reports making that servers ability to run your apps limited.
As far as designer goes you can install on a number of machines subject to your developer licences but place note you will not be able to use desktop as your source apps.
I've checked several articles (including ones published by you - Qlik NPrinting 17 Install Guide - for Qlik Sense reporting, for example) but I could not locate the information how to configure a Designer to be connected to NPrinting Web Console and to the NPrinting Server. I know I might be blind and it's Friday afternoon, but could you please point me to the right reference?