I would say it's worth the effort to give your main site a boost.  IMO the more content you can get on your own site that's relevant, the more Google has to base it's evaluation off you.  If your blog has alot of fresh content, that'll keep Google coming back for more!

Just because you have WordPress running your blog, doesn't mean WordPress has access to edit the files of the main eCommerce site.  You can install a standalone WordPress blog in examples.com/blog and have regular HTML files serving example.com

You can also takes steps to secure your WordPress installation on top of this.

If the comments above haven't swayed you from looking at density, SEO Quake has a plugin for Firefox that will analyze all the words on your page and give you density for each word and each phrase.  It may bring to light some phrases you weren't meaning to target, or show you a word you have much too high a density of by mistake.

Building links on highly relevant sites would be a good idea if you're willing to put in the time to make a useful post.  If the forum allows do-follow links, that will help you a little.  As part of a balanced link building strategy, they could be beneficial.

Any site that sells you thousands of links will be using an automated tool.  This is considered very spammy because the links have no value, they'll be on trashy sites, and on sites that there will be thousands of other links on as well.  The value a service would provide you wouldn't be worth the money.

Can you check to make sure those pages are still indexed by Google?  If the pages that were indexed are no longer indexed, then your canonical links have interfered with the ranking.