A supply voltage of 4.2 V would exceed the CC2511F32's absolute maximum rating for VDD, which is listed in Section 3 of the CC2511F32 datasheet, so it could damage the MCU if you are supplying that power directly to the Wixel's 3V3 pin without a regulator.
I am not sure why you think a regulator is necessary in order to get accurate potentiometer readings. If the potentiometer is powered by the same voltage that powers the CC2511F32 processor, then the raw ADC reading should stay the same even as that voltage falls. For example, if your VDD falls by 10%, then the voltage on the potentiometer's output would also fall by 10%, so the raw ADC reading should stay the same and still be a good indication of the position of the potentiometer.
Why are you pulling up P2_1 and P2_2? For low-power operation you would want to pull down those lines or just drive them low.
The radio consumes current when it is on, so you should consider turning it off if your goal is to conserve power. You might want to calculate the amount of current that can be saved by turning off the radio and how much it would extend your battery life.
All the information we have about turning off the ADC can be found in the CC2511F32 datasheet. It looks like there is no special bit available that can turn off the ADC, so it probably just turns itself off after it is done performing a conversion.
The code you posted looks like it will put the Wixel into sleep mode 2. To minimize the power consumption of the board while you are in that mode, I recommend referring to my earlier post on that subject:
Your code looks similar to the code in that post except there are some I/O pins that you are setting to inputs which were set as outputs in the code I posted. The reason I made those pins be outputs in that code was to prevent the extra power consumption that happens when you have an input that is at an intermediate voltage. If you want to conserve power, you should make sure that the voltage on all of your pins is either at 0 V or VDD.