I hate when people show up unannounced, but in this case, I am on your FIL's side. Since this isn't a traditional landlord tenant relationship, you can't expect the same courtesy another landlord would give you.
I understand the frustration since you are doing a lot to help out your fiance, but, no offense, it's not his parents' fault that he got in trouble for DUI. It's not right to be short tempered with them for stress they didn't cause.
Lastly, I am in 100% agreement with FMIL about the nachos. It's none of her business if you ate them in bed. But you left them there. If a pest problem arises because food is being left out, they will be the ones to pay for the exterminator, not you. I don't blame her for saying something.
Yes, it's annoying, but I don't think there is a while lot you can do in the situation.