While I'm not going to (at least explicitly) answer your title question of "How active should moderators be in closing questions?" as I feel that this is something that needs to gradually be worked out through time and indeed, questions like this on meta (we're all learning here, after all!), I'm going to attempt to address the various issues you've raised.
First, I'll go through your suggestions, then I'll go through the (current) comments on this question then finally, I'll explain my thoughts on that particular question you're referring to.
First of all, for your first two points, which I've briefly summarised:
- Only close questions single-handedly iff they are blatantly off-topic or spam
- If you feel that a certain question is a homework-type question, a badly formatted question or just "broad", please don't close them right away
There are (at least) two relevant 'frequently asked' questions on SE Meta that look at closing questions and are relevant here. 1: As a mod when should I vote to close content that is not flagged? and 2: Closing/Migrating question instructions/guidance (which is, ironically enough, closed - it's outdated due to explaining some old close reasons, but still has relevant points).
The most important part for this suggestion is Shog's answer to 1 which is essential reading when asking about mod-closures. While all of it should be read before continuing with this answer, I'm going to pick out a couple of points for the moment: "When you see a post you think should be closed, close it" and
If you see a poor question asked, you can quickly close it and either edit or request edits, and then re-open just as quickly once the problem is fixed - this works to prevent answerers from wasting their time writing answers to a question that will change while they're writing.
In other words, if a question needs improving, it should either be edited, closed quickly (yes, even mod-hammered closed), then edited (preferably also quickly) and reopened, you guessed it, quickly.
In your question, you mentioned "any action that you (the mod) take single-handedly will take 5 (that's a relatively large number!) people to undo". In this case, it should actually be "any action that a mod takes single-handedly can be undone as soon as the required edit is made", which leads on to the answer to 2 - under "What Does "Closing" a Question Mean, Anyway?":
Closing a question is not an inherently negative activity. It should not be considered so, by either those closing, nor the original poster (OP). Instead, "closing" a question should be considered a form of sequestering -- placing the question in a state of Limbo.
This in turn leads to your fourth point:
- We need more active editors and helpers/commentators than downvoters and closers.
Especially in light of the recent meta posts and blog, this is something that's very important. As such, it's worth over-emphasising: "Closing a question is not an inherently negative activity". What is negative is not (pleasantly) helping the OP fix the question to make it better (or not pleasantly explaining why it's not suitable for this site, even if edited). In other words, yes, you're right, we should definitely edit and use comments to ask for improvements (in a Nice way). However, that is not saying the we shouldn't close questions. Indeed,
Closing a question merely prevents new answers from being added. The question, and any existing answers, can still be edited, voted and commented upon
In other words (as I've already mentioned in my comment), if there is any reason for a question to be closed, closing a question is good as it allows for the question to be edited, without new answers being made, which, as this allows for edits to be made without invalidating answers, can actually lead to better questions and answers, which is what we want.
Taking a bit of a turn, for your third point:
- Even if you notice that a new question is a duplicate of an existing question ..., give it some time
Again, in the answer to Closing/Migrating question instructions/guidance, under the section (surprise!) "Exact Duplicate", it turns out that questions should be marked as duplicate when either "questions are, for lack of a better term, identical" or "if the answers from another question adequately answer the new one, or any answers posted to the new one are likely to be duplicates of answers from the old one". This does in fact mean that if a question is merely similar and isn't answered by an answer on the proposed duplicate, it's not actually a duplicate. You're doing the OP (as well as the potential answerers) a service by essentially telling them "look, this question has already got everything you want/need!".
You mentioned in a comment that "I personally would never go an add an answer to a 2 month old question, when I could possibly helped a new user by answering their current question". Why? Please feel very welcome to add answers to old questions. There are even badges for doing so. It's better to have a single question with the answers well collated, with several duplicates all pointing to that question, over having multiple duplicate answers spread throughout the site, with the best answer being hard to find. Not only is closing a question as a duplicate helping the OP by telling them that an answer to their question already exists, but it's helping future visitors by making it easier to find the more commonly asked questions and the best answer(s).
In other words, while I'm not sure exactly what you mean by "seemingly" duplicate questions, it's possible that the question(s) you're thinking of shouldn't actually be closed as duplicate(s).
For your first comment, this is exactly one of the reasons that mod-closing a question before anyone has answered can be a good thing - it allows for editing without invalidating existing answers, as there aren't any existing answers.
This (finally) brings me on to the question you linked. This question was closed (before there were any answers) by a mod, then reopened by five 'normal' votes. In this case, the mod-hammer worked exactly in one of the ways that it's supposed to (only, not what a mod-hammer is most often used for). That is, the question had the opportunity to 1. be edited without having to worry about invalidating existing answers while 2. allowing the community to determine if the question should remain closed. It just so happened that the community felt that the question is allowed as-is and was reopened.
The thing that could have been done better by the mods (as you mentioned) was explaining that this is why the mod-hammer was being used in more detail.
What could have been done better by the community was undergoing a more thorough process, using comments or meta, to edit the question to boil down the 5 questions, split up into 3 'main' questions, which was basically 2 heavily related questions into the actual 2 questions being asked, or at least, fixing the formatting to make it clearer that it was 'not too broad', although this is also something that the mods bear a fair amount of responsibility for as well.
Closing a question can be a good thing.
Duplicates are also a good thing. (although, don't ask for the sake of asking)
Mod-hammering a question closed has more uses than simply getting rid of blatantly off-topic or spam (or rude etc.) questions. One such use is to quickly close a question so that it can be edited if necessary while the community decides whether the question should be reopened or not.
However, editing a question is preferable to closing it.
We need to explain (in a Nice manner) why we're closing something when we close it, unless it's rude or spam etc.
We're still in the initial stages, so (as well as trying to expand and improve the site) everyone is still in the learning stage. Even once past the initial stages, we're still in the learning stage, so asking questions like this helps define and clarify what should be done and when.
Having said that, in the spirit of this answer, I believe that your question is fundamentally a duplicate of that SE meta question I linked and I merely expanded upon Shog's answer. As I've already said, duplicates are good.
P.S. Your P.S. really is a duplicate