I can't think what's causing the flat spot, unless it's the turbo actuator not responding properly - which is something your model has suffered from. They changed from vacuum to electronic actuation and mucked it up.
The actuator should normally cause the arm beneath to rise and fall a small amount - 3 to 5mm - at idle (remembering that "idle" isn't constant, but it varies as the fuel input varies so that the ECU can ensure the HO2S is functioning). If you can get someone to sit in the cabin and apply throttle to bring the RPM from idle up to 1500 and let it idle again (repeat) you should see significant movement pretty much in time with the load demand.
As for voiding the warranty, only if the fault is as a result of something that was (or was not, and should have been) done during servicing. If you've done the servicing as per the log book, Nissan cannot void the warranty - for example, if the timing chain starts to let go during the warranty period.
Where the larger effect of non-Nissan servicing can be found is AFTER the warranty period is over. Nissan have performed "goodwill" out-of-warranty repairs in the past - and they lean towards this if there's brand & customer loyalty - but also because they expect that the servicing (even if performed by someone far less qualified) is done in strict accordance with their regimen.
