This is due to your long backButtonText ("More Pascal RIBEN Galleries").
In Small Screen Mode (on mobile devices), the Back Button and Button Bar share the same horizontal space.
Space is reserved for the Back Button and, once this has been displayed, the remaining space is used for the Button Bar (which will wrap within the available space if necessary). This avoids the Back Button and Button Bar overlapping each other.
With your long backButtonText, there is enough space to the right of it (on your device) to display only one Button Bar icon (which is why it wraps to display all the icons in a single column).
If you were to reduce your backButtonText to something shorter (like "Home", for example) or use the 'home' icon (by setting backButtonUseIcon="TRUE"), then the Button Bar would have more space to display icons in a row.

If you really wanted to, you could change the icon used for the Back Button to a custom icon more suited to the page that your Back Button redirects to.
If you'd like to try this, please take a look at the Using Custom Icons support section in the Theming Guide.
The IcoMoon library might have an icon that you could use for your mailing list link, otherwise you could create a new icon yourself.

Yes. There's no need to change the backButtonUrl (the URL associated with the Back Button). Just change the backButtonText to something shorter than "More Pascal RIBEN Galleries", for example "Mailing List".