Thanks. I discovered that indeed pruning was the problem, but it didn't completely solve my MEF woes. I still couldn't get my plugin to load.
Here's the process according to my poor recall. Originally, I MERGED the interface assembly into the main assembly. This made the interface assembly invisible to the plugin. I then tried to EMBED the interface assembly into the main assembly. Although it found the plugin, it couldn't find the plugin's referenced assemblies which were merged into the main assembly. I tried several combinations and I couldn't get them to work.
After several different setup arrangements, I found that I had to embed the plugins references into to the main assembly but leave the interface outside of both assemblies. Magically, this works and the plugin is able to reference the assemblies in the main assembly. The resulting plugin is very small and references everything it needs from the main assembly. Ideally, I wish the assemblies were merged instead of embedded for better protection. Also, it would be nice to deploy two assemblies, main and plugin, instead of three: main, plugin, and interface.
Given the popularity of MEF, I would strongly advise that it is directly addressed in either the software or technical documentation. I more than happy to provide assistance in helping reproduce these issues.