Hybridization of oxygen in Nitrate ion and the location/bond of nitrogen's lone pair
The actual structure of the nitrate ion is symmetrical. It forms a trigonal planar molecule.

This means that the bonds between oxygen and nitrogen are all the same. The resonance structures are a simple way to explain that structure using two-electron bonds. In reality the "extra" electron is delocalised across all three bonds (this will involve the nitrogen lone pair and nitrogen will be sp2 with no free lone pair). 

The average bond order will be, as you calculate 4/3 but this doesn't, as you claim, make the molecule unstable.