You’re correct about Silicon - you could also mention that it has a macromolecular structure to perhaps gain a mark for that keyword.
Aluminium is not a simple molecule. It is a metal, and therefore has metallic bonding throughout the substance giving it a higher melting point than Phosphorus as metallic bonding is quite strong. Phosphorus is a simple molecule, with each separate molecule only held to the other surroundings molecules by weak van der Waals forces, which require the least amount of energy to overcome them.
To clarify, van der Waals forces are the same thing as London dispersion forces, but they are not permanent dipole interactions which require a polar bond to be present in the molecule. Whether you call them London forces or van der Waals forces depends on your exam board, I think. I usually just refer to the interactions by name. Also, the amount of electrons don’t have anything to do with the trend in these melting points, as each element exhibits a different typed bonding which is a far larger determining factor in their melting points.