For fire, yeah. I don't expect CAT5 to get warm with 700mA. Is that 700mA @ 12V? Or 700mA @ 5V (which is just ~300mA @ 12V ;) )
But if you have a wire you can use 2 (aka a pair) for GND and 2 for 12V. Or if you do expect a to big of a lost, you can use a boost converter to up it to 24V, transport it and then buck convert it to 12V.
For the digital info, the 12V is not a problem. All the noise it can pick up from anywhere because of the long cable is a wayyyyyy bigger concern.
I presume you are saying that if the remote 12V supply is left on while the local circuit is off, then it could drive both circuits in unpredictable and potentially dangerous ways.
Yes, as in, power the Arduino in a way it's not designed which can kill the Arduino.
But, I believe the ULN2003 has diodes that (I think) keep the 12V away from the rest if the circuit.
You never told us what you are driving. If it's only ULN2003 it's indeed not a problem. But a one power supply solution is so much more easy, more elegant and cheaper :D
And when driving the ULN, I would put a pull down resistor (of let's say 1k) on each inpout of the ULN [u]on that side of the cable[/u] to keep them off good to make them more robust against noise/interference.
As far as the wall wart meltdown, the meter was not effected. It still works just fine. It was not plugged it, it was using a battery. The wallwart had 2 prongs. Maybe it was faulty, or I did something and dont know what it was.
My money is all-in on the latter :D