Action potential changes in low sodium concentrations but constant osmotic pressure
High sodium extracellularly means an increased sodium concentration gradient across the membrane. This means there is a larger driving force for sodium to enter the cell once the sodium channels open at the start of the action potential, and hence a larger depolarization takes place increasing the action potential amplitude. The enhanced depolarization leads to an increased proportion of activated voltage-operated potassium channels that open upon depolarization. When more potassium channels open, more potassium will exit the cell, thereby repolarizing the cell to a larger extent, in turn stopping the depolarization step of the action potential faster. In all, the action potential develops quicker, as the depolarization is increased as well as the repolarization step, and hence the time it takes to execute an action potential is diminished. 

Your subquestion as to the small sodium conductance is not valid, as during an action potential it is actually very high.

To increase the action potential duration you can increase the re-polarization step by increasing the extracellular potassium concentration, thereby diminishing the potassium concentration gradient and prolonging the repolarization step.