Just a quick note that I have finally completed this round-trip journey, and everything went very smoothly, thanks no doubt to those who went before me. And many thanks to the OP. I checked in at PNH for the routing PNH-DOH-SGN and the agent didn't note anything of significance. She even tagged my bag through DOH to SGN, and gave me through boarding passes at PNH after showing my visa for Vietnam. All of my flights were originally scheduled to be operated on the ex-Virgin Australia aircraft with 1-2-1 seating and the fun little bar, but the first flights were on a standard QR 77W with 2-2-2 seating. Fortunately, I had no seatmate, so the flights were all fine. I was able to get two e-visas for Cambodia before leaving home, which made that process easier.

Again, thanks to those who have reported their experiences, as it certainly made my travel smoother.