I had the same questions and problems when I hooked up my Mac to the Oppo 105 for the first time. As you requested:
1) I agree that a higher quality USB cable will great improve the sound when connecting the MAC to the Oppo or Hegel. I am using the Synergistic Research Active SE USB Cable but you should be able to find something less expensive.
2) When you plug the USB cable into the Oppo 105, you need to tell the Oppo to use the USB as the input. There is an input button on the remote and you need to scroll through the list until you find USB. You should be able to see your input selections on the front panel of the Oppo. I believe that once you elect USB as the input, the Oppo will remember it the next time you turn the Oppo on. If you are switching back and forth between the MAC and the Oppo, you have to change the input to the correct source (CD, Video, etc). Since I played everything using my MAC, the Oppo kept the USB as the source input and I did not have to change it every time.
3) Yes, you need to use an external monitor when using the Mac Mini to see what you are doing. If you decide to use the MAC Mini, I suggest you purchase a small 12" monitor. I was considering the MAC Mini but since it requires an external monitor, I decided to purchase the 13" MAC Book Pro.
4) If you decide to use your MAC Book Air, I suggest you purchase an external hard drive to store your music on. As I noted above, I am using the LaCie 3TB d2 USB 3.0 Thunderbolt Series Hard Drive. It uses the Thunderbolt connect so it does NOT interfere with the USB cable going to the DAC (Oppo or Hegel).
5) You can always call Oppo Customer Service if you need help with the Oppo. Their customer service is terrific. I have called them several times and they get an A+.
Okay?