This is the way kids this age are they are just trying to figure things out hence they ask a lot of questions. I seriously doubt that this child is trying to undermine you they just want to know why. This is a skill they are learning in kindergarten... cause and effect. My 6 year old wants to know why I stay up late and she goes to bed at 8. I simply say because I am the grownup and you're the child. The kid will catch on just keep answering the questions... once they ask a question that you have already answered say what did I say last time make them think about it they will eventually remember. Gently teach the child how they should treat adults....Maybe their questions aren't answered at home who knows. I seriously believe it is better to have your childs friends over at your house knowing what is going on than to have them at someone else's house especially if you don't really know the parents. So make your house a place that the kids want to hang out.
Just a quick story my BF at about this same age had the meanest parents they always acted annoyed at anything anyone said except their own kids...of course! I once found a nickel on the sidewalk in front of their house and was so excited (that's a lot of money to a kid) I took it inside to show it off. The dad made me give it to him saying it was his since it was on his property. I was crushed. We stayed friends for a while until I realized I really wasn't welcome at their home and I didn't like the way that I felt when I was at their house. Sad to say that once we (me & my sister) started playing with other kids in the neighborhood my BF stayed home all alone with her Great parents but she got to have lots of TV Time