Ditto cleaning.  And the self adjust only happens when braking in reverse btw... had customers where this didn't work because they basically never used reverse due to parking habits... strange but true.

If your brake shoes in the Sentra are still thick, then all they require is a good cleaning once in a while. 

Realise most of the braking is performed by the front brakes in any regular modern car, so it’s quite natural for your front pads & discs to wear out before the rear pads/shoes wear out. 

It’s no secret that Nissan’s quality has nosedived after their merger with Renault, hence you find their parts wearing out sooner than the Mazda. With that said, good aftermarket discs are relatively cheap these days and it’s no big deal to replace them whenever you replace your pads. Depends on where you are & your driving conditions.

Rear drum brakes are supposed to be self adjusting, that is, when you drive in reverse and apply the brakes, a lever is actuated to move the shoes out.  If this mechanism gets crudded up as it always will, pull the drum and clean everything with brake cleaner, lube the shoe to back plate contact area and the lever mechanism and it should self adjust again.

That wouldn't have anything to do with it. The rotors on the Sentra are probably too thin or otherwise of poor construction and are warping due to heat. This might also be caused by the brakes dragging and causing the rotors to heat up.

Most modern drum brakes (that is, since around the mid-1960s) are self-adjusting but of course that mechanism can fail.

With regards to your front disc rotor warpage. 

All the contact points, mating surfaces, should be clean and fee of debris.

Caliper hardware should be lubed and move freely.

Wheel fasteners should be torqued to factory specifications.