My questions is can I set up my Phantom 2+ to return home in case I lose contact without installing the software. I run on Mac OS 8.5 and don't want to screw up my Mac by upgrading.

You won't be able to download the DJI assistant software on your MAC.

"Download the Firmware via Assistant software. Mac OS X 10.9 or above"

The answer is yes, an out of the box Phantom will be in "phantom" mode and RTH will be activated as standard.
Once you have a confirmed home point set, turning off the transmitter will activate RTH.

Now if you mean the NAZA Home lock and Course lock features, then No, you will need to connect to a PC and run the assistant to switch modes.
You must know someone with a PC or better MAC that can help you...