Question 1
Regarding A: use a voltage divider in parallel with the 8Ω resistor.
Regarding B: If you make sure the 8Ω resistor is being used with its ratings (provide cooling if necessary), you can just apply Ohm's law using the voltage found in A.

The Arduino will handle any unit conversion, RMS converting, etc as you already wrote.

Question 2
A 12-bit ADC gives you a higher resolution than a 10-bit ADC. Accuraccy =/= resolution. Accuracy depends on the resistor's tolerance etc.

Question 1
Regarding A:
Use a voltage divider in parallel with the 8Ω resistor. Your approach would influence the measurement.

Regarding B:
If you make sure the 8Ω resistor is being used with its ratings (provide cooling if necessary), you can just apply Ohm's law using the voltage found in A.

The Arduino will handle any unit conversion, RMS converting, etc as you already wrote.

Question 2
A 12-bit ADC gives you a higher resolution than a 10-bit ADC.
Accuraccy =/= resolution. Accuracy depends on the resistor's tolerances etc.