Let's not forget that Rick is a sheriff and sitting across a table from your enemy is nothing new to him. Getting the guy to talk, lie to you, tell you the truth by omission etc should all be in his bag of tricks, even as a small town sheriff. He went there in hopes of working out a truce, but knowing that it would be unlikely.Imagine if Rick has all the true info on the Gov from Merle, like the Gov has on him from Andrea. There is no way this meeting takes place if he hears the story of the cold blooded murder and looting of the troops that they did. Andrea has told him everything and this meeting helped Rick level that information playing field.Now--as to the fence and zombies. Go fix the outer fence and kill/get rid of the zombies in the inner fence. All it does is leave them that much closer to you and make your life harder to do anything. I assume that the Gov has someone on watch to check on their comings and going, learn their defenses and weaknesses etc and that person told him about their gun run that they took. Rick had the guns and ammo in a duffel bag, so it is more then likely that he took an educated guess about what was being brought in with that bag.