For RS485, the TX and RX pairs should be on seperate twisted pairs (RX and RX* on one pair and TX and TX* on on other pair).

If you parallel two types of line drivers, you need to have some way to disable the RX on each. Otherwise they will fight against each other.

Power over Ethernet does supply power over the same cabel as the signal. so as long as you take in account the voltage drop you should be ok.