Google Analytics allows you to setup what are called tracking URLs.  They can be used when you "tag" your URLs with tracking code that allows for GA to translate which medium is generating what traffic.

http://support.google.com/analytics/bin/answer.py?hl=en&answer=1033867 This is a tool that Google offers to help you generate your URLs.  There are also spreadsheets available online that allow you to create the URLs in Excel but they aren't always 100% accurate.  What you are doing is ultimately creating parameters that allow for custom campaigns to be setup within Analytics.  This allows you to monitor more than just the clicks, but allows you to generate custom reports that are more insightful and provide better data.

Once your URLs and Campaigns are created and running smoothly, you will be able to look at the data of the multi-channel funnels.  This feature allows for attribution to be given to the digital and non-digital channels that don't always get the credit for the conversion.  There is a lot of data can be pulled from these models and they really help to better allocate resources if needed.

I hope this all makes sense and Google actually did a very good job of explaining these resources on their analytics guide.  I would take a look there as well to get a better understanding before diving right in.

In my opinion no.  The canonical is telling search engines that there are pages with similar content that do exist, but one of them is the one you want to be crawled for the information and content.

Best practices suggest that you only have one canonical per page, but I am unsure as to the penalties associated with having more.

I am still confused as to why you couldn't use tracking URLs to achieve your goals because it allows you to see which site is referring the traffic without having to create multiple URLs.  Tracking URLs are different from analytics tracking that you have installed on your sites.

David,

Something else you may want to try are tracking URLs.  If you are utilizing Google Analytics, you can setup tracking URLs that allow you to know which website the clicks are coming from, but also allows you to point users to the same URL/content on the website.

There are a lot of templates out there that can help you to get started using them, but once you do, it makes it a lot easier to track what mediums are driving what traffic.  If you want to get really spiffy, you can start tracking what traffic drivers are creating the most conversions and also start to utlize multi-channel funnels.

If you are interested I would be more than happy to help you get started.

Hope this helps!