1. Before this exploit you had to unlock the bootloader to get root. Also you need to unlock it to get custom kernel. I am not sure about custom ROM.
2. Yes, a few page in is a step by step guide, look for it.
3. You do void the warranty, but you can unroot it if you haven't hard-bricked it.
Device: Sony Xperia Z2 (D6503)
Bootloader: Locked (unlockable)
Root status: Yes