Is there any way to use Komplete Kontrol in standalone mode using the S25 keyboard controller?
Yes, but you'll need to download Komplete Kontrol 2.9.6 rather than 3.0.3 as the S25 is only available as a MK1 keyboard.
You can get it here - Komplete Kontrol 2.9.6 - Last VST2 supported version — Community (native-instruments.com)
Mike