How can I open a new window from within the running instance of kate? On the commandline I can use 'kate -n', but from the inside both ^N/file-new and ^O/file-open open new tabs and I found no way to change it. New sessions also open in new tabs. kate 17.12.3 - not the latest.

I think it's not possible. I've tried to take a look on the documentation https://docs.kde.org/stable5/en/kate/ka ... -kate.html and also in the Shortcut settings (from the Settings menu), but I didn't find any way to open a new windows directly from kate.

You can open a new window from the menu (press the Windows key -> search Kate -> press Return) or by adding Kate to the panel

Sure it's possible. You could do it like this. First, make some simple bash script. Like so:
#!/bin/bash
kate -n
Put it somewhere and make it executable.
Open Kate. Under tools>external tools>configure. Create a new tool. Make the executable point to the script. Give it some icon and, if you want, put it on the toolbar. Done.
https://imgur.com/a/Gz87sc3