Current measurement over a fairly large range
This is not going to be easy.  The normal answer is to use a high side current sensing chip, like the INA16x series.  Although I haven't checked the datasheet, I expect that this won't yield 16 bit accuracy.

The first strategy would be to see if you can re-arrange the circuit to do low side current sensing.

That failing, you probably need to float the A/D with V1.  If V1 was always at least enough to power the A/D, then just a negative linear regulator would be sufficient.  However, since V1 can go to 0, you have to supply some other power.  Tie the positive output of the special A/D power supply to V1, then opto-couple the digital lines in and out of the A/D.