If you set $wgDefaultSkin = 'minerva'; in LocalSettings.php and visit Special:Preferences (when Beta features is not installed), you get a fatal "MWException from line 151 of includes/Preferences.php: Global default 'minerva' is invalid for field skin".
I checked $wgValidSkinNames inside a mid-execution hook and Minerva was included, I also checked $wgSkipSkins, Minerva wasn't there.
If instead you make Minerva the default skin by setting`$_GET['useskin'] = 'minerva';` directly in LocalSettings.php, the Preferences section works fine.
I'm using MediaWiki 1.27.0 and the corresponding MobileFrontend branch.