I'll let others answer your first 2 questions because I don't have a whole lot of time, but for your other two:
3) HO roles typically include a suicide lead (sometimes two) to get hazards up immediately and doing the best to shut down removal options. HO also stacks as many powerful pokemon as possible (usually setup attackers as well) to pressure the opposing team.
4) As for who fits these roles, it kinda depends on your theme. If you want Veil HO, obviously you need Alolan Ninetales. For Rain, you want Pelipper (followed by stuff like Mega Swampert, Tapu Koko, Kingdra, Hawlucha, etc). For Trick Room, you want sturdy setters like Uxie or Cresselia with powerful abusers like Stakataka or Crawdaunt. Webs needs a setter like Shuckle or Araquanid with stuff like Mega Pinsir being common abusers. For just general HO stuff, typically suicide leads like Lando-T, Excadrill, and Greninja are used, with pokemon like Dragon Dance Zygarde, Double Dance Magearna and Bisharp capable of applying lots of pressure, using their high bulk (esp behind veil) to setup in the former 2 examples and the ability Defiant to deter defogging away hazards in the latter example.
As you can see, HO can go a lot of various ways and I hope this helps at least a little!