Electrike/Manetric learns Thunder at level 36. Plusle/Minun learn Thunder at level 37. Then there's always Pokemon from the previous generations that can learn it as well. Like Ampharos, Pikachu, Elekid, etc.
Manectric can't learn thunder fang in sapphire because that move doesn't exist in the game only in diamond, pearl, platinum and any other game after those you can teach manectric thunder fang.
Luxray can learn thunder fang.
The following Pokemon that you can easily get to learn Flash in Sapphire: Beautifly or Dustox Lotad or Lombre Oddish Gloom
Houndoom cannot learn thunder fang by leveling up. He can only learn thunder fang is when you breed a Houndoom and a pokemon that can learn thunder fang example ( Luxray )
Zapdos will learn Thunder on it's on sometime around level 60.
Castform does not learn Thunder as it levels up. Thunder is a TM that you can teach it. (In Pokemon Emerald, Ruby, or Saphire, you can get Thunder in the Lilycove Department Store.)
No.
It can learn it via the TM but not by level.
No
You can only clone Pokemon in Pokemon Emerald. Not Sapphire and Ruby. To learn how to do it, search it up for a guide.
When you learn to use the force.
No.