1. You can't enchant an already enchanted weapon, so no, you can't cast it on a magical weapon (in the FAQ thread somewhere).
2. Yes, you can cast OOG spells before your first game, but Enchant Weapon isn't an OOG spell, so you've nothing to Embed before your first game.
3. No, only on an arrow (so one-shot only) (again, in the FAQ thread somewhere).
4. We've yet to see a wizard with a bow, as it's easier to get a decent casting number on an offensive spell (which potentially have a longer range). Here, people have been boosting Fight to avoid being shot.
The other problem with your suggestion is that is two turns worth of activation to get one shot off. T1 cast enchant (along with a move if you wish), then T2 you can shoot - meanwhile, a bonedart/elemental bolt caster has had at least one shoot in return, potentially two if they got initiative.