Current sensing at μA accuracy and ADC nonlinearity
1) Yes. Go back to your original goal of determining optimum fix interval. How is having ten 32-bit ADC in parallel vs. one 24-bit ADC going to affect your decision. Is your fix time in nano, micro or milliseconds? Most likely you don't need such ADC precision to get the optimal solution.

2) Yes. same as question 1. Look at the ADC you selected, at the desired 100SPS the intrinsic noise is 3.5 uVpp, this will probably not work at the low end of your range. So an auto-range or multi-range with average precision setup would be better that a single range with high precision.

3) To take advantage of 32-bits of resolution, the PCB layout will play an important role in minimizing noise but just looking at the component selection, you chose a 1% sensing resistor for instance with 0.1% ones readily available, I think you have other problems to consider.

4) This has been answered in the comments. While for high voltages the least-significant bits of the ADC are a small fraction of the conversion value, for tiny voltages these LSBs are significant.

For what is worth, on similar applications where long battery life is the final goal I have used Coulomb Counters (such as Linear's LTC4150) to measure the power consumed. Configure the system with different transmission times, let it run for some time and compare the total consumption for the period.