1. Yes, it effects the watk/eatk portion.
2. Weapon element is overwritten by endow/convert/etc.
3. Cannon ball overrides everything. Also, cannon ball element mod is applied to the watk/eatk portion, AND THEN AGAIN APPLIED TO THE TOTAL DAMAGE. Whenever you can get a cannonball elemental advantage, do so. It is insane.
4. yes, you will do less damage, because the watk/eatk portion will miss. The satk portion which is not effected by weapon element is neutral and will hit the monster.
5. Can't parse the way you asked that. Say base damage is 9k, 4k from eatk, 5k from watk/eatk, using cannonball with 200% ele mod vs target. 5k * 200% = 10k, 10k+4k =14k. Now apply the 200% again, 28k damage.