The clicking from the dash is a dodgy stepping motor for the climate control, don't worry it still works and is ok.
The antenna should stay up when the boot is popped and some cars lower the antenna so the spoiler dosen't bend it.
Dont worry about the splash guard or indicator causing problems the indicator is sealed.
The parking assist antenna is probably dodge so don't worry, the switch you refer to is for the operation of your Automatic transmission. Power selects gears faster and snow makes the car take off in 2nd gear to reduce wheelspin in slippery conditions.
Skylines do come out with a 5 speed auto, never seen one in person but know it to be a fact. the best way to find out would be to see the transmission code stamped on the platr in the engine bay and check that against other Skylines or a Nissan Dealer may have info regarding this? enjoy your ride!