Wouldn't all of these important events from 1973 been mentioned in X1/2/3.
Still doesn't explain how Charles was at the airport in the original timeline.
There is something that i'm missing.
No because the events that have happened in this movie create a new timeline.
There are two separate timelines now.
Timeline #1)
One where the original movie exists. When Mystique kills Trask, it creates the timeline of the original movies. The important events such as Professor X and Eric saving Mystique DON'T happen in this timeline because it's the timeline where Logan isn't sent back to alter it.
So Professor X stays depressed and later on gets his crap together, Eric remains locked up until he escapes on his own somehow and Mystique kills Trask at that board meeting as she intended too. Then Origins of Wol, X 1-3, The Wolverine all happen in that order.
At the end of The Wolverine, Professor X has already came back to life after the events of X3 (as you already know, from that crappy X3 After Credit explanation) and with Eric, meet up with Logan warning that Trask has been building the Sentinels due to what Mystique did in the 70's. Yes, it's not 100% explained how Charles came back to life after X3 but it's basically hinted that he brought himself back to life through the use of his power. Remember, they also don't have the memories of what happened in DOFP because those are different Charles and Eric. The Charles and Eric in X1-3 never came together to save Mystique in the 70's, they did there own thing.
Once Logan is sent back in time in this timeline, this creates a new timeline the moment he wakes up in that girls apartment.
Timeline #2)
In this timeline, with the mind of #1 Timeline Logan in his 70's body, goes and convinces Professor X and Eric to meet up and stop Mystique from killing Trask. This is where the events of DOFP happen. Magneto does his stadium magic, Charles convinces Mystique to not kill Trask, Logans body is retrieved from the water by Mystique disguised as Stryker. It is still unclear if Weapon X happens or not in THIS timeline. Now with his job done, #1 Timeline Logan's mind drops out of the 70's body in the water and goes into limbo. It cannot go to his future body in the original #1 timeline because it's not there anymore due them dying from the Sentinels so his mind automatically goes to this timelines body in 2023, where Professor X kept his promise that he made to Logan on the plane about building the school back and recruiting him and Scott and etc. X1-3 and The Wolverine don't happen in this timeline, it happens in the other timeline, not this one.
In this timeline, the events starting from when Logans body is retrieved from the water to when he wakes up at the school is unclear to us and to the mind of #1 Timeline Logan. Why?
Because once he died in the water, his mind reverted back to his 70 mind, just like the way it did when he was having head trauma from seeing Stryker in the conference room. From the point where he was retrieved in the water, he went on life in his 70's mind, not knowing what just happened. The only people that know of Logan going back in time is Professor X, Magneto, Mystique and Beast in THIS timeline.
Finally in the year 2023, #1 Logans mind enters this timelines body and he's confused because he wasn't living THIS body from the time he died in the water to the time he woke up in 2023 because his mind was in limbo. That's when Professor X says he has to catch him up with what has happened and that he has been waiting for #1 Logan's mind to come back to #2 70's Logan's body.
It may sound confusing but I hope that clears it up a bit for you jye.