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We actually just put this in the navigation so it should be a bit easier to find now.

Don't change a URL just because it is htm instead of html. That change will have ZERO positive impact.  ZERO.

Did you make changes to your site? I.e. when ranking and bounces rates changed, was it (looking at analytics) at a specific time that you made structural changes to your site? have you looked in google analytics to see where the traffic decline is coming from? i.e. have you looked to see if hte decline is in organic search or adwords traffic? just google? just bing? or across the board. looking at these things will give you an idea as to whether this is a wholesale issue, or perhaps a technical issue with one service. your open site explorer metrics look decent.

don't use canonical this is not what it is for, use a 301, redirect, and make sure that all your links in your site point to domain.com. when people link to your site, you want them to always link to the same url, using a canonical will not hide the index.htm, and there for you will get links and book marks to the wrong url