Sounds like a dead pixel. The person you talked to is just trying to weasel out of fixing your problem.
Your screen protection would not have any affects on the LCD, unless the screen protection was installed incorrectly (like using way to much pressure or heat).
IAmSauro :
The LED 'bulbs' right under the screen where you have the white spot have most probably died. and under NO circumstance can you repair this on your ownI'm afraid. Go to a Sony Service Centre,and fast.Its still under warranty,and if I'm right,it'll take a load of money to replace it.
The LCD can most certainly be replaced, but you can't fix the LCD with a dead pixel.
The LCD replacement is quite simple, cheap, and quick.
A guide can be found here;
https://www.youtube.com/watch?v=jmxni3_CBjw