Stitcher re-distributes your media (they download it once, then serve it on their own internal network), there's no matrix we can provide you because of this. There are some folks who do not like Stitcher for this reason. You can treat Stitcher like a video sharing site for audio. Almost all podcasts are available on Stitcher, except those who have requested to be removed of course.
Some folks fear that Stitcher could someday add advertising to the content similar to Pandora without any compensation back to the podcaster.
You are correct regarding uniques being a larger total than the platforms/clients/countries, and this is because there are applications/hardware that have no user agent or country information available. Also important to note, the platform/client/country information is calculated from the unique statistics, and the overall downloads is an adjusted total based on university and corporate networks that share an IP across multiple computers.