Only 2 is true.
Here is why:
1- bonds strength of ionic bonds are measure via |lattice energy|=|(q+)x(q-)| / |(r+)+(r-)| . Since the charge of mg is 2+ greater than Na which is only 1+, Mgcl2 has stronger bonds.
2- it is correct, this is one of the characteristics of transition elements.
3- while it is true that most covalent compounds have lower melting and boiling points than ionic compounds, covalent compounds that have strong covalent bonds such as diamond or SiO2 have much higher or comparable melting and boiling points to ionic compounds.
4- Not true, an example of a covalent compound that can conduct electricity is graphite. It does so with the since free pair of electrons each carbon has between the sheets of hexagonally bonded carbons. Graphite however, only conducts electricity in 1 constant plane and direction.