There exists a second, nearly identical plugin folder under:
C:\Users\User\eTeks\SweetHome 3D\Plugins
containing many, but not all, plugins.
This duplication is confusing and complicates troubleshooting.

That's strange! It should never happen.
Try to make an completely fresh install as I've described and take a look into Sweet Home 3D folder after you run the program at least once.
It should make all folders with lower case(e.g. plugins NOT Plugins).
With some upper case could be the PluginManager folder if you've installed PluginManager-1.5.3.sh3p under plugins.