I think you don’t need any resistors, they will only attenuate the input signal, and you would not want that. Connecting all the C7s together on the one hand and the C6s together on the other hand and wire them to the 2 connectors in the jack respectively should be fine. I’m assuming the sound source has a relatively low output impedance. Is that the case?
Mixer headphone out is intended to drive a low impedance output, though headphones vary quite a lot — 32 to 600 Ω or so, with 32 Ω common for consumer headphones. This module on the other hand has a 3.9k resistor on each channel as well as a 1 µF capacitor, so a much higher impedance. Give it a try but the results may not be what you’d like. You’d probably get better results by taking the input from the mixer line output; of course that output would have to be split between the headphone splitter and whatever sound system the mixer feeds into.
Yup, but with five of them in parallel it’s something like 780R so not a million miles off, although I do wonder about what the extra capacitance is going to do…