Consider the following: "Lo conocí durante cuarenta años". "Conocer" is atelic in this meaning, so by nature takes time; in your concept, it should go in the imperfect. Well, it doesn't. It is the closed time interval "durante 40 años" that forces the preterit because "conocer" is an atelic verb in this context. If we had used a telic verb, the imperfect would be conceivable:"Durante 40 años solíamos vernos todos los domingos".
I think I finally understand it. It would be better to say that the closed time interval
allows one to use the preterit. If the time interval were not closed, that is, if we didn't know when the state began or ended, then we
couldn't use the preterit, and we would have to use the imperfect.
So, there are two cases where we can use the preterit:
(1) When an event occurs at a given moment in time.
(2) When an activity or state endured for a
fixed interval in the past.
The imperfect applies to cases:
(1) Where the activity was habitual.
(2) When a state or activity continued for some period of time.
A. Él quería ir a Europa, pero cuando llegó la guerra, decidió ir a los Estados Unidos.
B. Siempre quiso ir a Europa, y lo hizo después de la guerra.
In (A), we have a state that continued for some time, before being interrupted by a sudden event. It's an open interval, because even though we know when his state of wanting ended, we don't know the moment in time when it began. So, we can't say exactly how long his desire lasted. Since we have a continuous state without a fixed time interval, we
must use the imperfect. The first rule you quoted applies.
1) The interrupting action goes in the preterit and the interrupted action goes in the imperfect
In (B) the person's state of wanting is also continuous over time. However, use of the word 'siempre' allows us to conceive of a fixed interval. Since 'siempre' is all inclusive, it can be presumed to have a begining and an ending. This allows us to use the preterit.
And, this is the point where the argument really begins. Does the closed time interval
force us to use the preterit, or is it the simple fact that we have already used the imperfect for case (A), and so we simply need to use another grammatical construction for case (B). That is, if we had to use the imperfect for one case, and the preterit for the other, isn't our choice dictated by the simple fact the preterit fits (B) better than it does (A)?