All the sharing icons and other icons appear overlapped.
This is a known problem which has already been addressed and will be fixed in the next version of Juicebox.
Forcing Large Screen Mode for all browsers and devices is, indeed, a compromise (which is why Juicebox usually defaults to using Small Screen Mode on small screen mobile devices).
Unfortunately, Juicebox has not been designed to do what you want it to do (show the Back Button on the Small Screen Mode thumbnail pages).
If you link directly to your gallery (as you currently do), the user can always use the browser's own back button to return to the original web page.
However, if you were to embed your gallery directly in your web page (rather than just use a link to the gallery's index page), a Splash Page would be displayed (in Small Screen Mode) and when the Splash Page is clicked or tapped, the gallery would open in a new page (like a link). However, the difference in this scenario is that there would be a 'Close Gallery' button on both the thumbnail pages and the main image pages which would take the user back to the original web page.
Maybe could be interesting to link the gallery title back home
Ordinarily, you would be able to make a Gallery Title into a link by using HTML formatting as described in this FAQ:
How do I add HTML formatting to the Gallery Title or Back Button?
However, there is currently a bug whereby a Gallery Title is not clickable in a Small Screen Mode thumbnail page.
I have logged a bug report with the developers and this should hopefully be fixed in the next version.
With Showkase is the same?
Juicebox galleries function exactly the same whether they are embedded in Showkase pages or any other web pages.
However, Juicebox galleries are embedded into Showkase pages (rather than just linked to), so when a gallery is opened from a Splash Page, there will always be a 'Close Gallery' button visible (on both the thumbnail pages and the main image pages) to take the user back to the original web page.