Hi there,
1. if you view any page and select Customize from the Admin Bar it should remain on that page, likewise if you navigate to a new page whilst in the customizer. Maybe a ‘coming soon’ plugin or something is breaking that functionality ?
2. Yeah font is too large to displayed correctly centred.
REM units are based upon either the browser default size ( generally 16px ) or whatever size property set on the HTML element. Unless you define a different REM value for mobile screen sizes its not going to differ.