Players not playing in character 'correctly'
As a GM, I'm familiar with this sort of thing. Primarily players insulting the person hiring them, or the leader of the guard, or the queen, or what-have-you. How do I handle that? Simple: they don't get paid as much, or don't get the job. If I were to outright insult my prospective boss in an interview (which is basically what the old man in the tavern is doing: interviewing you), I'm fairly certain I won't get the job. I hold true to that in D&D. Insult the queen's weight? Why should she hire you? There's plenty of other adventurers to hire. If there is no-one else they can hire, or they're desperate? The job's pay just dropped, and the Persuasion/Negotiation/whatever check to raise the pay just got tougher. This is usually followed by the person hiring the PCs making some comment about it; "I'm certain a man with a mouth like yours can find better work elsewhere", or "a hundred gold is more than enough for the soap your tongue needs".
If a PC uses OOC knowledge to get an advantage, and I catch them in the act, I reprimand them or punish them. I'll either call it out (which usually results in an apology), or I'll silently change the stats and tell them after. Mention that you can keep back-peddling from the zombies and kill them with ranged weapons because they have a lower movement rate than you? I just increased it so you're even. That last one did happen. When the plan failed and the zombies easily caught them again, the offending player asked how they could move so fast when the Monster Manual gave their movement as lower than they had moved. I pointed out that looking up the zombies in the Monster Manual is metagaming, and so I changed the stats as punishment. The player hasn't done it since.
As for PCs never being afraid of things, I can't control that as a GM so easily. D&D 5e has Inspiration, which allows me to reward those who play their character, but some characters are the typical Captain America-types who never surrender/back-down/flinch. I'd be interested to hear if someone has a solution for those characters, other than OOC saying "no".