opened SH3D and imported Furniture.jar.sh3d

Guess you meant "imported Furniture.jar.sh3f" or "imported Furniture.sh3f".

Is it correct that no new Furniture.jar will be created in the \lib\ directory, but the imported file is saved in C:\users\xyz\Sweet Home 3D\furniture?

Imported libraries are saved in C:\Users\xyz\AppData\Roaming\eTeks\Sweet Home 3D\ furniture

If so, will I need to remove that one before making further changes?

Not necessarily, as long as you keep the same file name, the imported library will be overwritten. No need to relaunch Sweet Home 3D too, unless you directly edit the files found in C:\Users\xyz\AppData\Roaming\eTeks\Sweet Home 3D\ furniture

Also, there were many items missing in Furniture.jar, like outlets etc. How can I change those ones?

You can merge this library with other libraries found at 3D models import page, using the button showing a + sign. You can also simply import these libraries without creating a larger library.