We usually use all 4 pairs to power the unit, this does allow you to not only power the radio but 802.3at devices through the PoE port.
You can you any combination of Polarity the N000000L142A uses the following:
PIN 1,2,7,8 ==== V-
PIN 3,4,5,6 ==== V+
As for using 2 pairs, for 35W you can use two pairs, however if you are considering 2 pairs and 26 AWG instead of the more usual 24 AWG you need to be aware of voltage drop issues over cable lengths.
We would recommend you use at least 24 AWG cable in this configuration with Ethernet cables.