Nur wirksam wenn geschlossen
Why is »nur« positioned before »wirksam«?
Because in this position »nur« refers to »wirksam«. The sentence means:
The only case when the door is effective, is when it is closed. When the door is open, it is not effective.
Lets look at this sentence without »nur«:
Wirksam wenn geschlossen
When the door is closed if will be effective. But it is not really clear, if it is effective or not when it is open. Without our knowing about doors this sentence allows the meaning, that the door is always effective, open as well as closed.
So the word »nur« makes clear, that only closed doors are effective. Open doors are not.
Would it be possible to move it into the second part, along with »geschlossen«?
Generally spoken word order is very flexible in German, but when you shift the word »nur« to other places in this special sentence (that btw is not a full sentence, but an ellipsis), you get wired results.
Lat's have a look on them:
Wirksam nur wenn geschlossen.
This sentence is grammatically correct and has the same meaning as the original sentence, but - different from »only« in English - In German you rarely put »nur« behind the word that it refers. So this place for »nur« is allowed, but sounds strange and is rare.
Wirksam wenn nur geschlossen.
The long form of this ellipsis is:
Die Tür ist wirksam wenn sie nur geschlossen ist.
It is hard to find a comprehensible interpretation for this sentence. The only one I did find was this (but maybe you will find another way to interpret this sentence):
There are some states that the door can have, and those states have a natural order corresponding to the amount of effectiveness. One of this states is »geschlossen« (closed). But there might be an even stronger state (maybe »zugemauert« = »bricked«). This sentence says, that all states that are as strong or stronger than the state »geschlossen« are effective.
So this version is also grammatically correct, but has a wired meaning. (Nobody is going to brick the door to make it more effective)
Wirksam wenn geschlossen nur.
When I would read this sentence, my first thought would be that it is the result of a bad machine translator.
The meaning is similar to the previous version, but additionally you have the strange place of »nur« behind the word to which it belongs, which makes this sentence even more wired.