So what exactly is the 'real' Sunnah in terms of appointing a leader? Our Prophet (saww) nominated many people to important posts during his 23 years of Prophethood. How did he make appointments? Any examples from Quran as to how to choose a leader?
You just answered your own question, there is no hard and fast rule for appointing leaders from Quran or Sunnah.
prophet appointed a huge variety of people ranging from muhajireen to ansar to ex-enemies as leaders of expeditions.
But Prophet himself was the supreme leader at that time so there was no need for someone to organize the affairs of the Ummah , as it later became an issue after his death.