... there is no config.xml file included in the export.
There should always be a 'config.xml' file in every gallery generated by the Lightroom plugin.
If your gallery folder does not contain a 'config.xml' file, then something is wrong (and the gallery will not be able to be displayed in a browser or opened in JuiceboxBuilder-Pro). Try reinstalling the plugin from scratch to ensure that all core files are present and correct. Also, check that you are not exporting your gallery to a folder with read-only permissions.
I deduce that if I want a responsive gallery then I should create it manually in JuiceboxBuilder-Pro, correct?
The Lightroom plugin creates a gallery very similar to JuiceboxBuilder-Pro (with no functional differences with regard to responsiveness).
With both applications, you can select the gallery size (Gallery Width and Gallery Height) and the 'index.html' page generated will contain the gallery on its own at the specified dimensions with no other content on the page.
Gallery dimensions can be percentages (of the gallery's parent container) or fixed pixel values. The default gallery size for both the Lightroom plugin and JuiceboxBuilder-Pro is 100% x 100%.
If you use percentages, then the gallery's parent container in the 'index.html' page generated by the Lightroom plugin and JuiceboxBuilder-Pro is the <body> tag (the entire browser window).
For more information on how to make a gallery responsive, please see this forum post.