Thank you for that explanation, it makes sense. I didn't know that the jar libraries in the plugin directory couldn't be shared, strange. I can do that easily with C#.

I think I would have made a different choice: a "headless plugin" with the extensions for 3Dview interaction and install that together with any plugin that requires it. If it's already there it will just be overwritten/ignored keeping it a single install.
The difference with a regular library is obvious: a plugin is loaded by Sweet Home 3D, a custom library by the plugin. I wonder if there's another way around it.