1 - might have read that about the FR-S halogen lights, BRZ all had HID's. They illuminate the road just fine, though the '18's LED lights will be a bit brighter.
2 - The auto-dimming feature of the Homelink mirror works great. Can't comment on the actual Homelink part as I don't have a garage
3 - at the end of the day it's your money, get what you want.
Though don't automatically rule out a '17 if you can save a bunch over an '18. The Car Play and Android Auto could be added via a head unit easy enough, and the one touch lane change can be done via a
flasher (plus some bonus features like strobe hazards!)