1 & 2. It is important when a person is mounted on top of you that you keep your elbows in and on his thighs so he doesn’t slide up on top of you. You must keep your opponent over your hips and not over your chest. This will also allow you to absorb the pressure of your opponent laying on top of you and putting weight on you.
Your legs can be posted or laying down depending on what you want to accomplish.