This is a two-part question.
1.) Is there a known adapter / power injector to operate condenser microphones with the audio input of the ADC. Otherwise does anyone have experience with building their own? For example like in this schematic: https://upload.wikimedia.org/wikipedia/commons/c/c0/Tonaderspeisung-T12.svg
2.) Can you directly connect headphones to the RCA outputs (with an RCA to 3,5mm jack adapter) or is the output impedance/power not suitable and you would need a headphone preamp?