Seeing how Imperials typically adress events of major importance -such as the Warp of the West, Miracle of Peace- to actions the Divines, it is qeustionable if Akatosh -or Shezarr for that matter- made the Amulet of Kings. Only when enhough of those have been take down the barrier becomes penetrateable. Nu-Mantia intercept offers an explanation by giving Nirn several Towers that reinforce the 'natural' barrier. That doesn't explain the better half of the Second Era when nobody had the Amulet. But seeing how Reman and Tiber and half a dozen pretender kings have no significant relation to Allesia it can be assumed that anybody bound to the Amulet can activate the barrier. The 'Trails of Saint Allesia' and 'The Amulet of Kings' both mention that an heir of Allesia has to wear the Amulet.
The claim that the Emperor has to be a Septim is false, although the notion can be blamed on their constant rule in the last Era and proganda used to inform the population about this particulair part of History.
