To your questions:
1.- Is an additional license: there is NPrinting Designer and NPrinting Server (that includes the On-Demand features)
2.- NPrinting offers a full integration with MS Office, while QlikView Report Generator allows the user to create his own reports within QlikView. Which one to use? I see use cases for both. Actually NPrinting distributes QlikView reports as well.
3.- QlikView Publisher does not produce excel, powerpoint, pixelperfect, etc. while NPrinting does. In terms of integration, so far NPrinting and QlikView Publisher are two different products, having two different services and managment consoles. However, NPrinting does not reload natively QVW files, nor distributes or reduces. Again, there are use cases for both or either of them.
Since you are a partner, please reach out to your partner manager or closest Qlik office and ask more detailed questions that we could tailor to yours and your customer needs.