It looks like you have gotten a lot of sound advice. I just wanted to offer a suggestion of what I do with the why questions. My daughter loves to ask why and demand a reasonable explanation. It didn't take me long to get into the habit of answering the why before she asked the question and now it is part of my habits. IE. If I was in the dining room and my DD came in and I was concerned for her safety I would ask that she leave the room but making sure I give her the reason. "DD please don't come in here right now. Mommy is moving the furniture and it could fall and hurt you. Just for now, go into the living room and when I'm done moving this I will let you know." Now this isn't to say that she doesn't get stubborn or choose to ignore the information but I repeat myself one more time if that is the case and if she stays on the path of Why my next move is to inform her that I took the time to explain it all and she needs to take the time to listen. I ask her to tell the answer and when she does ... she accepts it. Something magical happens as she explains it back to me ... acceptance.
Also ... my DD was never the typical toddler with tantrums but when she started having them I came to realize she had them for a reason. We were not paying attention to her demands for more independence. When we rectified this the tantrums stopped immediately. Now that she is 3 1/2 she doesn't have tantrums but more arguments with us. She has great logic too but I have come to discover that I can sit down with her and have great conversations and she will definitely point out things she feels is unfair and why. If she is valid I will 100% agree with her and we make agreements to change it. I totally agree that children should have a say and parents should listen but I also think it can quickly get out of hand if the parents take it to the level of just giving in to the child. I always tell my child that I am open to discussion but she has to be able to argue her point and you would think a 3 1/2 year old wouldn't understand that but she does and has since she was 2. The big difference now is the complexity of her argument.
