Can I use Charms on already completed encounters?
This was my 1st week of doing LFR and I killed all 17 without getting any loot.
Can I go back in and use my charms at more attempts at loot?
Yes. You will only get one roll per kill though, but you can kill as many times after the first of each week (where you get two rolls).
Yes. If you have charms you can go back into LFR and use bonus rolls for a chance at loot, even if you've already used a bonus roll on that boss a previous time.