As Flaming Foxx said, he had no control over the Mists.
He was just the first living mortal to traverse them.
Except the norn havrouns, kodan Voices, gods (let's face it, they aren't immortals), demons, forgotten, human ancestors (brought by the gods from through the Mists), etc. etc. etc.
And what understanding? Nothing said he knew anything about the Mists - at least no more than your average norn havroun who are able to traverse the Mists with the Spirits of the Wilds' help. He just knew how to open portals to and from the Mists (and from the Mists isn't that hard, seeing how the Rift which he traveled through a lot (as it contains the Hall of Heroes) is basically a portal hub, able to easily connect to any time and place).
But nobody other than him was able to get to the Rift.
You mean nobody other than him and everyone sanctioned by the Zaishen or ghosts of the Tomb of the Primeval Kings. Nobody except the hundreds if not thousands of combatants who fought for honor and glory within the Hall of Heroes (which, as mentioned, is in the center of the Rift) until the rise of Zhaitan (come to think of it, why was a portal in Lion's Arch necessary when Zhaitan rose, considering there's the portal in the Tomb of the Primeval Kings - sure, it was corrupted by Abaddon, but he's been dead for 100 years since then... unless it is/was still contested by demons at that point in time).
And if my theory of the Realms of the Gods being part of the Rift is correct (and it seems highly likely given the evidence which I can provide if requested), nobody but those people and all the adventurers who went into the Underworld, Fissure of Woe, and Realm of Torment. Which lore surrounding Dhuum's return and Godslost Swamp make sound to be a lot
of people (especially assassins).
What's interesting to note is that the Rift is said to be a place with no true passage of time, thus if one is able to master the perspective of passing time they could travel through time. That is, I would assume the situation is that people go to the time they want, however due to the perspective of time constantly passing, even if it doesn't really, then you will instinctively leave however much time you perceive as having passed (e.g., take a watch into the Rift and although time's not passing, the watch keeps going, and when you return with the watch, the time of the watch matches the time you return, even though you're capable of leaving the Rift before you even entered it).
If Odran managed to do this, we could see him alive during one of his journies. Would be funny and we could say "aren't you dead?" "dead? Nonesense!"