When you found the cart, did you talk to both drivers? If you talk to Fred but not Luther, that quest log entry may not be cleared.
In either case, once you have the letter from Victorio, you should be able to talk to Marcus, which I think you must have already done, in order to get the letter from Pedro (he doesn't turn hostile until that part of the quest is advanced).
Lt Robin should have a dialogue option after you get Pedro's letter.
By design, only 2 of the 3 people in the quarantine area can be given a cure. Other than cheating to duplicate a potion, you can also Pickpocket one back to re-use (level 2 required) or kill one of the cured NPCs so they drop the cure potion.
