#1 - That's probably too many links to have in the sidebar. I would make a link to a "Useful Articles" page and categorize the articles there. This does depend on how you want your visitors to flow through your stie.

Yes to #2. Link with keyword rich anchor text too.

#3 - only if those articles help your website accomplish its goal(s).

1. Go to https://adwords.google.com/ko/KeywordPlanner/Home, click "Enter or upload keywords to see how they perform"

2. Copy/paste list or upload a file, set your targeting preferences. Press "Get Search Volume"

3. Click on the "Keyword Ideas" tab, download

Hope that's what you're looking for!

Cheers,
Oleg

Depends. If the people who would use the Facebook pages have different needs depending on the location, it may be better to have a separate page for each location. However, if all the pages would pretty much be the same (you would post the same updates to each and the questions from customers for each store are pretty much the same), I would stick with 1 brand page. Its easier to manage and it is more authoritative than having 12 separate pages.

Have a client in the exact same situation. Check to see if you are currently getting traffic for terms that would be specific to having separate pages (e.g. "50 blue bic pens" versus a more general "bic blue pens"). If you don't, then you should canonical to one page. If you do, I'd keep it as is and work on diversifying the product pages more.

Why do you believe that the second page is indexed? What was your search query?

Long as you have the canonical tag set up, you should be fine. If they link to the url with the feedburner variables attached, it is the equivalent of linking straight to your actual post page.