Soundflower is not sending multi-output sound to all devices
While I don't fully understand what you're doing and what goes wrong I can't tell you how to configure Sound Flower the right way. But I know an alternative solution.

Using Audio MIDI Setup, to send Audio to multiple devices:

Your Mac has Audio MIDI setup installed by default. By selecting the + and choosing 'Create Multi-Output Device' you can send audio to both your headphones and to soundflower (which sends it to OBS if I understood you right).