Is this a delay circuit? / Which resistor is responsible for load impedance?
R3 simply connects R1,R2 to ground when phantom power is switched off, and the C simply provides some decoupling on phantom power. No delay, and I would expect quite a thump when switching on phantom power.

R4,R5 provide a 4.4K load (leg to leg) on the microphone, or 2K2 on each leg - and you are absolutely right that this impedance is further reduced by R1,R2 which are effectively in parallel (for AC) with R4,R5.

And phantom power is always supplied from 48V in series with 6.8k on each leg - it's a de-facto standard that studio mics are designed to. It's not primarily current limiting (though it does that too), but to keep the impedance on the audio signal reasonably high.