Mainly I am trying to control the following aspects of the UI in mobile only"
Juicebox-Pro uses one set of configuration options for both Large Screen Mode and Small Screen Mode so, unfortunately, it is not possible to configure each screen mode differently. Additionally, certain configuration options are Large Screen Mode only (such as captionPosition and imageTransitionType) and these cannot easily be changed at all for Small Screen Mode.
There's more information about Screen Modes and how Juicebox adapts to different devices and screen sizes here.
What you might like to try, however, is force the gallery to always be displayed in Large Screen Mode (no matter what device or browser is being used to view the gallery) by setting screenMode="LARGE" (in the 'General' configuration options section).
If I could see these things in-browser on desktop I could use Inspector.
If you want to see the Small Screen Mode version of your gallery in your desktop browser, temporarily set screenMode="SMALL".
Alternatively, if you use Firefox, you could perhaps try using 'Tools -> Web Developer -> Responsive Design Mode'.
Another suggestion would be to use an extension such as User Agent Overrider which, as the name suggests, overrides the user agent string for your browser (to identify your browser as something that it's not). You could select 'iOS / Safari 10', for example, and your gallery should be displayed in Small Screen Mode (as if on an iOS device). This is not an emulator but it should be suitable for examining the Small Screen Mode version of your gallery in your browser's developer tools.
There should be similar options and extensions for other browsers.
I hope this points you in the right direction.