externally powering the Arduino questions
I am guessing that you are thinking of dividing down the battery voltage from 7.2V down to 7.2*(4.7)/(4.7+1.5) = 5.5V. This is generally a bad idea for a few reasons.


The battery voltage will not be a constant 7.2V. When they are fully charged it could be even higher. As they discharge they could drop significantly and your Arduino POR (power-on-reset) circuit would potentially shut the board down before the batteries are fully discharged
The batterys' internal resistance would probably drop the output voltage considerably once the resistors try to draw 1A of current. It might even collapse the voltage to some unusable value. 
Even if the battery internal resistance is very low and even if the Arduino is not drawing much power (or even off) you will always be drawing ~1A of current from the batteries. This will rapidly drain your batteries and will also heat them up from their internal resistance. Not to mention your external resistors better be able to handle > 5W or so. Conclusion: lots of unnecessary heat and power loss.


To answer your other question, assuming the servo is low power and the Arduino power supply is up, you could power it from the Arduino 5V supply (not your batteries).

If you need to use batteries to power an Arduino, I would highly recommend a power regulator to produce a stable output voltage to power your Arduino.