your dealer is lying to you. courtesyparts is an online store of an actual nissan dealer (like your dealer) called courtesy nissan and they sell genuine nissan oem parts. i know cause im a pretty regular customer of theirs.
Courtesy Nissan - Nissan Dallas - Nissan McKinney - Nissan Mesquite - Nissan Lewisville - Nissan Plano - Texas, TX
im not sure if nissanpartzone is an actual dealer or not, but ive bought maintenance parts from them in the past and they too sell genuine nissan parts.
dont listen to everything your dealer says. they're out to make money from service and they will charge you an arm and a leg for a 15min job. you can source your own metric screws too with standard philips or hex head.
no need to use the security screws.
just take one of the security screws off while you wait for your steering lock to arrive.. bring the screw to your local acehardware and just find the same thread and pitch, length.
sorry to hear about the other problems. in general the Z is a very reliable car.