High side current monitor for bidirectional high voltage rail with low current
Why don't you use an isolated power supply (e.g. Murata, Meanwell, Aimtec) withstanding the desired (from 500 to 5.3 kV available) and an opto-amplifier like mentioned AMC1100 or Avago ACPL78? Input voltage is usually +/- 100 mV to 200 mV, so for 15 mA you put 6-10 ohm.
For 0V and input reference: for the Avago pin 3 (-IN) shall be put to 0V (GND of supply) and the signal can swing those 100/200 mV beneath 0V. Drawn input current in the range of 1 uA, so not disturbing your reading. Supply current not negligible, yet.