If all you plan to do is drop your links in social bookmarking sites, you're probably just wasting your time.  It's not likely that these links will be worth anything to search engines - even if they are followed links.  The time you spend building these links could be spend developing great content, or working on other link-building opportunities that would be more worthwhile.

As far as blog comments go - if your goal is to get backlinks, it's not a good idea.  These links are nofollowed and too many of them will look spammy to search engines.  If you are commenting to engage with the content author, that's a different story.  I often comment on other sites to engage with the content author and start a conversation.  I sometimes choose to include my website as a reference, but I often choose to leave it out.

Yes the presence of BingSiteAuth.xml or the lack-thereof doesn't really give you a definitive answer, but it does give you a little insight into the situation.

There is really no way to know for sure if they have activated or are using Webmaster Tools accounts unfortunately, unless you have access to their email accounts.

I am not aware of anyway to determine if a site has implemented Google Webmaster Tools or not, as they could have verified it with by uploading a randomly named file, or through Google Analytics.  I would say if they are fairly large site and have Google Analytics (which you can determime by viewing the source code of the site) they probably have GWMT as well.

To determine if Bing Webmaster Tools are in use, you can go to http://example.com/BingSiteAuth.xml (replace the domain name) and see if the file exists.  If it does, then they activated Bing Webmaster Tools.  If the file does not exist, it either means that they never activated Bing Webmaster Tools, or they did and were smart enough to delete the file from their server.

Hope this helps!

Moz has published a lot of posts on Link Building: http://moz.com/blog/category/link-building most of them have some solid advice.

As Chris stated above, many in the SEO community feel that guest blogging will be the next target for a Google update, so while short term it may pay off to put some time in and find a site to post your content - in the long run your content might be better off being posted on your own site.

I've always felt that if my content was good enough to be posted on some other site, then I would rather it be on my site in the first place.  This is unfortunately the only real future-proof method, as it's hard to say what will happen to guest blogging as a link-building tactic.

Using tables in your HTML will produce a poor mobile experience.  Because the world is moving more towards mobile everyday, we can't ignore it, and neither can search engines.

I don't think search engines will punish you for having table tags in your HTML, but if your site is consistently producing high bounce rates from mobile users - you're not really winning are you.

Google tends to "think" for you when running searches.  They often assume like words mean the same thing, so running a search query for "funny images" and "funny pictures" will likely produce very similar (if not the same) results.  You should take this to your advantage, rather than worrying about it too much.

I see this used a lot when I search for "web developer" and get results for "web development" and "web design."  The keywords are bolded as if Google assumed that is what I meant.  I don't think it's a problem, just something to account for in your keyword research.

The post excerpt is likely the first paragraph or so of your post, and often times doesn't include your target keywords and is not a good preview of what the post is all about.

You should tailor your meta description to potential readers.  Give them a reason to click through and read your article.

Your Open Graph description should also be tailored for social networks to promote sharing.  This description is likely different than your meta description and probably much different then your post excerpt.

Why not try to avoid the problem altogheter?  I assume the below line is site-wide:

If you remove the index, follow meta tag, your pages will still be indexed and followed.  Then allow your authors to decide on a post-by-post basis if they want to noindex or nofollow the page.