+1 on Krafty's advice. The internet is a great place for having a whinge but you hardly ever see people getting angry enough to post about how good their car is.
The catch can can't harm the engine at all because it's only catching gases vented from the top of the engine and not interacting with the motor in any mechanical way. A larger intercooler can't do too much damage either - all it can do is cool the incoming air stream more effectively. This might change the algorithm in the ECU for the most efficient burn because the air in the chamber would be slightly denser but I can hardly think that's going to cause too much damage to the motor!
By "ECG mod" I think you mean EGR - now that's not going to void the warranty and I doubt Nissan will say much about it - it's an emission control thing, nothing more, and unless the engine dies from melted pistons they'd be hard-pressed to reject a claim. Note that EGR cools the combustion temperatures in the throttle mid-range ONLY, never at idle or full throttle, and full throttle is where you develop the dangerous heat - and the EGR valve is closed then anyway, so blocking it won't make a difference to the top end heat production.
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