Not sure, but the update cycle of the plugin seems not to be the same like the main application JuiceboxBuilder!?
The plugin requires additional code over and above the Juicebox core code (to integrate Juicebox within Lightroom and to provide the user with an interface in the Lightroom Web section).
When a new version of Juicebox is released, a new version of the Lightroom plugin will be released.
However, if any bugs are found in the Lightroom plugin code, an interim version may be released (between Juicebox releases).
Does the plugin cover all the features of JuiceboxBuilder and also the features included in the PRO version?
The Lightroom plugin features individual controls for Juicebox-Lite configuration options and Juicebox-Pro options can be entered as text into the 'Pro Options' text area (see Step #4 in the 'Upgrading to Juicebox-Pro' instructions on the Lightroom Plugin page for details).
Do I need to buy the PRO version of JuiceboxBuilder or is the Lightroom plugin completely separate from that?
The Lightroom plugin comes bundled with Juicebox-Lite so is completely free and can be downloaded from this web page.
If you were to purchase Juicebox-Pro (to use Pro Options) then you could upgrade the plugin by following the 'Upgrading to Juicebox-Pro' instructions on the page I linked to above.