Following up on 1, you don't strictly need to divide to 3v3 -- you can divide to lower than that and just adjust your code accordingly. That way you can get something up and running quickly until you are able to get the required resistor values to do the 3v3 division.
I would also second the INA219 / INA226 suggestion, as those modules would do the voltage detection for you over I2C. Additional benefit is that if you happen to miswire things, the module is first in line and would probably give up the magic smoke before the pixl :P (I killed a number of INA219 modules because of reverse polarity while testing some unprotected 18650 cells).
Speaking of shields, you'll also need to be careful as many Arduino compatible shields assume 5v and may further reduce power to their components (if you wire the pixl such that 5v = 3v3) or might sink more than 3v3 on a pin (if you wire the pixl such that 5v = vin). I just set 5v to 3v3 by shorting the pads, and test the voltages with a multimeter, just to be safe.
Great thanks for pointing me at the INA219/226 - I've ordered 2 x 226....
Also thanks for the comment about Arduino 5v shields. Noted.....
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