I largely agree with the answer by 1muflon1, but let me give a slightly different perspective...
There have been a number of questions on this site where it is fairly clear that the questioner has a strongly held opinion on a topic, because their text seems more to be presenting an argument than asking a question, in extreme cases because it reads like a rant. Such questions are generally unwelcome. Conversely, questions formulated in a way which suggests the questioner is open to different points of view and willing to learn are much more likely to be welcome.
Having said that, the "pros v cons" formulation is just one way of formulating such an "open-minded" question, and perhaps invites opinions rather than economic reasoning or evidence. Other and perhaps better ways include asking for (in the context of economic theory) "reasons for or against" or (in the context of applied economics) "evidence for or against".
While questions on economic policy are certainly not off-topic as such, they should be carefully formulated to encourage answers focusing on economic considerations rather than personal or political opinions, and reasonably focused so as not to be too broad. You make a good point about avoiding an assumption that everything is measurable, but formulations in terms of reasons or evidence can also avoid that assumption.
I'm not sure I understand your item 3, but re item 2 there are many areas of policy which may not be primarily driven by economic considerations but nevertheless have economic effects. Congestion charging to limit traffic in cities is an example. Questions about such policies would I think be on-topic provided they are focused on the economic effects.