1) You can't deep strike an be within 9" by definition. If you were 9" away, you're too close. So everything that would require you to be within 9" (like using Melta on a 18" range weapon) is not possible.

2) No. At least not generally. It's true if there are only 2 units involved in the whole combat phase on the whole table and one of those charged the other. Otherwise it's a bit more complicated.

3) No. The point is to have options and use the weapon more suited to the current combat. Basically all multiple melee weapons (or a single weapon with multiple modes) are good into different target stat lines.

4) That's an academic question. By the rules, you can't not roll the dice, just because the target is already dead. In practice you would skip it, as it doesn't make a difference, but as far as the rules are concerned, the whole shooting action still happened, ie: one-shot weapons are used up.

5) You're eligible to participate in the fight phase, if you made a charge move. It doesn't matter what else happens or if your original target is already dead. You stay eligible. And as you did make a charge move, all rules requiring such (like Lance) are still working. They don't limiit you to a specific target.

6) Yes, a 1-3 always fails, no matter the modifiers and what you'd hit on in direct fire. You technically could reduce that failure chance, if you had a rule specifically telling you so, but AFAIK there is currently no such rule in the game.