Could Cat5e be used for RS485
Short answer: yes.
That should work just fine.
It is quite common to use CAT5 for RS485.
More detailed:
RS485 requires a common ground reference for all devices. Yes, you can use spare pairs in the cable as ground reference. Proper shielded cables provide better noise immunity. But it is not by any means necessary for RS485 communication to work. Depends on your environment. Lots of high power electrical motors and welders around? Then you may want to use shielded cables.
Depending on your exact cable length requirements, you may want to consider thicker gauge CAT5. Up to ~100m/300ft any CAT5 should do the job. Beyond that, the resistance of the copper conductor can start playing a role in attenuating your signal. Thinner conductor=higher attenuation. RS485 cables designed for 1000m/3000ft have really thick copper wires.