Question: I can't seem to find where to configure this option. Backspace unindent only works when using hard tabs, but should'nt this work since it works on other Scintilla based editors (eg Scite)?

Answer: 
The solution I've found, which works exactly like Scite's solution does, is a little plugin from Virtual Roadside named [npp\_tabs](http://www.virtualroadside.com/blog/index.php/2012/12/03/notepad-plugin-to-enable-unindent-on-backspace-key/). Highly recommended - after three separate attempts, it's the only thing I found that worked how I wanted.
npp\_tabs seems not working anymore.

Use ExtSettings, which is available through PluginsAdmin.

Website:
<https://sourceforge.net/projects/extsettings/>