The reasoning behind the present progressive representing a future event
I disagree. I see it as the same of the use of the simple present for the future ("I leave next week" is also valid). "I'm leaving next week" in no way implies that "I'm leaving today" in some sense; in fact, it has the opposite implication.
To me, what it seems is that the use of phrase specifying the time, such as "next week," establishes the reference frame of the sentence, and the present interpretation takes place relative to that reference frame. It's like "Imagine that it is next week. At that time, the following statement will be true: "I'm leaving.'" Kind of like the use of the "narrative present" with a time label attached, like in a script or something
Next week: I'm leaving.
This explanation also has its problems. We don't usually say things "I'm leaving last week", even though it's possible to imagine narrating an event in the past. I think this may be because narrative past exists and is naturally used often with past-tense markers.
I did find an example of present + "last week", though, which is maybe similar:
Funny how I say I'm leaving last week to come back and find this place gone to hell and not just my name being used but your and Gregg making endless false accusations that I'm guilty for it all.
ulTRAX reply to Modavations • 5 years ago - comment on "Small Biz Owners On Job Creation", http://onpoint.legacy.wbur.org/2011/12/07/small-biz-owners
How I would explain this is, narrative present can be used to describe past events as well, but it requires more context (a longer passage of narration) since by default people assume narrative in the present tense is non-past, since it could be but isn't marked for past tense.
Probably, there are a number of people who would find problems with my explanation, and there might be evidence against it that I don't know of. The fact is, native speakers don't really know this kind of thing automatically. People just use the present tense in these contexts because it sounds natural.