Really thought out ideas and you're making some valid points, but I don't believe that Elite 4 members will have strictly monotyped teams like Gym leaders had. They might be more centered on abusing their fields and covering their weaknesses. They might focus on at least two types that benefit most from the terrain, like for example Laura could have both Grass and Bug types, or El and Anna could also additionally have Fairy/Psychic or Fairy/Dark respectively. I would basically expect more balanced teams from these trainers.