It really sounds like every single storm cloud in the world has figured out where you live and is sitting over the top of you!
Ok, let's start with indication of gears. Mine - normally - shows P when I first turn the car on, then flickers 'R' and 'N' then 'D' as I drop it into Drive to move off. I don't press the O/D button on the side of the shifter at all, just the large release button so I can put the car in Drive. As I start to move off, there is no indication on the console that I'm in any other gear than Drive. If I stop the vehicle (or if I'm moving slowly, like under 20km/h) and I put the car down into 3rd, 2nd or 1st, the dash will indicate 'D' -> '3' -> '2' -> '1'. It will only register '4' when the shifter is in 'Drive' and I press the O/D button. Is that how yours works? If not, you could have a sensor problem in the gearbox. This sensor is driven directly by the shifter in the cabin but the sensor reports to the TCM inside the valve body of the gearbox which then reports to the instruments. I'd imagine one possible cause for this is TCM harness damage, but it might also just be the TCM hasn't been programmed correctly. It's a job (Nissan say) that only Nissan can do and from my own experience they don't always get it right.
Gear changes occur when the TCM is set to do that, based on inputs like engine load, engine speed, vehicle speed, driveline load - much of this is obtained by the TCM although the ECU does report engine load to the TCM (the TCM can sense engine speed because that's the turbine speed on the gearbox input).
Has the gearbox been serviced?