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The pagerank won't update. What you have now is what you'll have (probably forever). Good thing is that pagerank doesn't count. Your low DA/PA is due to the few links or low quality links you may have. If you want to increase it, you'll need more links (from high PA pages of course).

Shaqd I have done this, although I am not familiar with the canonical method you mentioned. Social share counts are tied directly to a URL so if the structure of a URL changes at all the old count will be attributed with the shares. You can create a custom social share button that pulls the data from the old pre  url structure change. Mike King has an awesome post about how to do that here (which is what I believe you are referring to with the og: url propterty?): http://searchenginewatch.com/article/2172926/How-to-Maintain-Social-Shares-After-a-Site-Migration  You can read a thread where he and I discussed it here: http://moz.com/community/q/preserving-social-shares-through-url-changes His method works.

I'm pretty sure it was on the product page you messaged about. Not that there is anything wrong with marking up several things on a page, but I'm just saying perhaps one of those other areas (e.g. pharmacy) is where the incomplete markup is instead of the product markup.

It really depends on your developers and your budget. I do development and SEO, so this is how I would handle it. On searches that are returning just one result, I would put something in place to see how many results are returned, if it is only one result returned, in the head of the page I would set the canonical url for the search page to the actual page that is being returned as the result. If more result is being returned, you can handle that in many different ways. One way would be to create a pseudo category out of the results page. I would use this sparingly and only for popular search terms. But you could have an extension written for your site that can give you some on page control of the text, the url, the meta areas, and things like that. I wrote a module for a platform I use a couple of years ago that does something like it. http://blog.dh42.com/search-pages-landing-pages/ You can get the gist of the idea by reading about it there, but that is one good way to handle a limited number of them to get them to rank better.  I would not do it with every search result though, you might get a penalty.