Mustafa Kemal was only one of many talented leaders available, and eventually everyone rallied behind him, as he was the most politically talented. If not, it would have been someone else. I'm always amazed that everyone just takes for granted that Mustafa Kemal was the only person in the whole Ottoman Empire who had any idea what he was doing. Most of his post-WWI military accomplishments were the work of other men, as he was busy with the political situation. In particular, Ismet Inonu was a critical military leader, as were a large number of others.
The Greek occupation of Smyrna was hopeless; they simly did not have the military or economic resources to hold the city, and the Turkish majority in the hinterland would not have tolerated Greek rule, which was genocidal in nature. Pontus was never offered to the Greeks, nor would it have been remotely possible for them to acquire it.
The Armenians had no chance whatsoever of holding onto any part of Turkish Anatolia, as by the end of WWI, there were no Armenians at all in the region. In OTL, the Armenian invasion was so feeble that the Kemalists didn't even bother to fight it (left defense to local militias) until the Greeks were checked.
Istanbul would have gone to the Turks, as Mustafa Kemal was if anything more cautious than other leaders and the Entente didn't have the political will to fight it out.
What type of government followed is much more up in the air, as a secularist republic was very much Mustafa Kemal's vision. More likely underanyone else, I think, would be a constitutional monarchy.