If Sturdy Javelin is on a ranged character, can I do 2 damage to any enemy, or must it be the defender?

Nope, you can throw the Javelin in any direction to hit any enemy.

Question regarding interaction between Backup Bag and weapons which can be scrapped during attack (e.g. Sturdy Javelin and Multi-Missile Pod). If I choose to scrap the weapon and it's saved by Backup Bag, can I choose to scrap it again during the same attack?

No. It scraps as a result of a triggered ability, which does not trigger again. You would have to somehow attack a second time.

If an opponent attacks me with a Sturdy Javelin on a ranged character and I reveal a Sabotaged Armaments I played last turn, how do the cards resolve? Does the Javelin scrap and deal two damage before the secret action takes effect or does the secret action scrap the Javelin before the two damage is dealt?

The attacking player will scrap the Javelin to do 2 Damage. Because attacker effects resolve before defender effects.