So is there any way for the Arduino to measure its own Vin?
that alternated between 5.073 and 5.051 every few seconds...although it seemed odd that it would change between those two values
If a voltage divider is the only way to measure Vin, then I assume that the reason that voltage divider is necessary is to reduce the voltage being measured to less than 5 volts so that the Analog pins can detect it properly.
What ever you do you can't put a voltage bigger than the supply voltage into an analogue input or you will damage the chip.
that alternated between 5.073 and 5.051 every few seconds.
But then I would have to know the positive rail voltage to know what voltage value was being divided (as well as the resistor values, of course).