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I also wouldn't discount the number of businesses that have previously been burned by so called SEO experts and were left with a bad taste for the industry.

Based on everything I have seen from the direction Google in particular is headed, I would bet on it being less and less of a factor as time goes on as far as search results are concerned.  However, that does not negate the benefit that can be achieved from the confidence a person may feel for clicking through to your site.

You can actually use the competitor ranking to your advantage.  By forcing you to focus on more long tail keywords further down the purchase cycle, you can actually end up generating better leads than focusing on those broad keywords such as "web design boston."  Find the niche you can fit into and exploit it.

If the items are going to be changing frequently then I suggest not adding the microdata to the home page.  Think of it in terms of you want the microdata for an item to be on the page you want most associated with the item for search purposes.

Its seems like you have a pretty sound plan.  To me what you are proposing doesn't invoke what Google is trying to prevent.  They are not advertorials and are indeed relevant to the sites on which they would be featured.

I think the best course of action is to pursue relationships rather than one-time guest postings.  Think like a consumer when evaluating sites to approach about guest postings.  If you as the consumer wouldn't find the site relative, reliable, or credible, then you probably shouldn't include it in your guest posting outreach.