does aluminum oxide float on molten aluminum and by what mechanism?
A chunk of $\ce{Al2O3}$ would sink in molten aluminum. However, $\ce{Al2O3}$ usually appears in smaller pieces (down to microns), with irregular surfaces, probably filled with air ($\ce{N2}$) from when it was cool. A mix of metallic aluminum and $\ce{Al2O3}$ could allow aluminum to sink thru the solid particles after melting, allowing the particles to rise above the surface of the liquid. $\ce{Al2O3}$ has a very high melting point.

The mass of solid particles would have some very weak structure, and some buoyancy by trapped air. The surface tension of aluminum is very high (~900 dyne/cm) , so it might very well account for the covering of the metal by a heavier substance and the non-wetting of the oxide.

This is reminiscent of floating a needle on water. 

In the electrolytic production of aluminum from $\ce{Al2O3}$:


  The electrolyte is a molten bath of cryolite ($\ce{Na3AlF6}$) and
  dissolved $\ce{Al2O3}$. Cryolite is a good solvent for $\ce{Al2O3}$
  with low melting point, satisfactory viscosity, low vapour pressure,
  and density lower than that of liquid aluminum (2 vs 2.3 g/cm3), which
  allows natural separation of the product from the salt at the bottom
  of the cell.


References


https://link.springer.com/article/10.1007%2Fs11663-999-0108-4 
https://en.wikipedia.org/wiki/Aluminium_smelting