In a survival island campaign I’m running as a GM, my players managed to sneak into a biochemistry Laboratory where an evil company were experimenting on animals, making super soldiers, and generally being very evil. When they entered the holding cell area, they found some prisoners, but also a guard who was watching over them. Fortunately they were disguised. One player tried to convince the guard to let them talk to the prisoners. When I was about to have them roll for persuasion, another player tried to deceive the guard into letting them inspect the plumbing in the cells. Then yet another player starts yelling at the guard, trying to convince them that it was now somehow the player’s shift as a guard, even though the guard arrived there literally a minute before the guard’s arrival.
Getting a bit confused, I decided to make everyone roll a presence check, which on average everyone passed. As a result of the conflicting instructions and requests, the guard basically broke, getting incredibly confused and was unsure what to do. When one of the players continued badgering him, I asked for another presence check, which when it was passed, the guard just shot himself since he was unable to deal with the extreme amount of pressure the player was putting on him.
After stealing the keys and starting to talk with the prisoners, the player who badgered the Guard to death felt remorse, and tried to resurrect the guard. The players were all level 2 and the player hadn’t invested a lot into creation, but I decided to just make the DC very high (30 in this case) and see what happened. When the player failed the check, I decided that the Guard was resurrected, but was severely mentally handicapped, since the player was not proficient with his powers. When the player tried to fix this further, he only managed to convince the Guard that he was somehow his father. Eventually, the players left with some of the PCs and left the guard to physically play with the broken computer which he inevitably broke when he was left with it.
Learning how to deal with PC’s comes with time.
In the case of multiple PC’s all giving conflicting information and orders to an NPC, I would have said out of game " are all of you saying all these things at the same time." and if they said said yes I would have had the guard most likely call for reinforcements. I usually run my games as lifelike as possible. If three people showed up and started giving you conflicting orders and statements you would not believe them no matter how convincing they were because the guy next to him is hindering him.
I typically try to get players to do one thing at a time as it doesn’t make sense for multiple people to be acting on one thing. (That is not a 100% statement of course) But again just imagine 2-3 people yelling questions or orders at you at work, people that you dont even know. You would most likely get a boss involved at this point or ask someone else if you should be doing what they are telling you. I doubt you would kill youself over it.
But in the end, the point is to have fun, did your Pc’s enjoy the game if so then you’re good. Sorry if I ranted.
This requires assertive GMing. Show them who is boss 
What I do (I had something like this happen before in OL) is have them all role presence the person with the highest roll is the person who the guard listens to if that person fails resolve the action and move to the person with the second highest and so on.