When speaking to @Samwilson during all hands he commented how MobileFrontend was creating problems for the global preferences work. When reviewing the state of the MobilePreferences code, I concluded that it's probably not necessary any more.
I suggest we load the core JavaScript on this page and remove our override in MobileFrontend. This will mean less code to maintain, we're unblock Sam's work.
The biggest issue with rendering the desktop preferences page on mobile is the horizontal menu at the top of the page (which does not scale well to a mobile skin) but given we do not link to preferences on mobile, making this horizontally scrollable should be an acceptable compromise.
QA steps
Visit https://en.wikipedia.beta.wmflabs.org/wiki/Special:Preferences and https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:Preferences and verify the forms are functional (not necessarily usable/mobile friendly)