Q:

How does a lurker gain reputation to receive privilege for commenting? 

I have been reading Stack Exchange for years - mostly not logged in, and have never posted (this is my first).  I'm simply not very talkative, and very good at googling. 
Recently a few occasions have come up when I've wanted to comment on a question.   I wanted either to ask for, or provide, clarification which did not warrant a new answer. 
How do I gain enough reputation to comment without:

asking questions which have already been answered or that I can figure out on my own, or
posting my comment like as an answer (i.e. being  rude)? 

A:

You have a reputation of 8 now on Stack Overflow; you need 50 to comment. 
You could do that by;

Correcting the grammar of either questions or answers.  To do so, click the edit link by the entry.  Rephrase poorly expressed needs.  For many, English is not their native tongue.  You need 21 more approved edits to gain the 42 more rep you need to make it to 50. 
Answer questions.  Each time another member upvotes your answer you receive 10 rep; you'll need 5 upvotes to gain the 42 rep required.  

If you get to 200 rep on any one Stack Exchange site, you automatically get an association bonus of 100 on every site, opening your ability to contribute freely on all sites. 
Think about the kind of answers that you upvote and answer likewise.  
Asking sensible questions that interest others is worth pursuing; your question here being a fine example. 

A:

Another way would be a "Q&A" type of question.  
As you noted yourself, you do have questions you already know the answer to.  However, these questions may be of broad interest to other readers. 
Therefore, you might post such a question and answer it yourself! 

A:

The web site is about content, and the reputation system is specifically made to incentivise contributions to the site.  It may sound harsh but people who don't contribute content and don't edit others posts are generally not very useful to Stack Overflow (more than as readers, readers are important but do not need incentives!),  so they don't get much reputation.  Why should you be able to comment if you are not going to provide answers anyway? ( Again, it may sound harsh, but don't take it personally).  Comments in general, even clarification comments, are just noise in the end. 
So, start asking or answering questions, or edit others'.  Start to contribute and you will get privileges in no time.  You are very much welcome to help create a great site!