I usually don't offer too much training advise, but, here goes. I think you have a potential serious situation developing . Have you considered that when you put the "dog away" after the dogs barks and shows aggressive tendencies you are make the dog aggressive. The dog feels her boldness and she might believe the behavior is what you want. In other words she "wins" , she has been encouraged to bark at the door, possibly praised for doing it or encouraged. The possessiveness comes with the behavior, ie; food, personal space, individuals. A 9 year old in the dogs head is a little adult, not a small child. I know plenty of horror stories of Labs biting for many reasons and severely injuring children, adults, (one field champion Lab at a field trial severely bit his owner handler, his handler went into shock) .
Having said all the above some of the most dangerous dogs are dogs that aren't taught to bite , vrs police service dogs ,military ,sentry and various security dogs that are taught to bite , hold appendages, etc. A untrained dog that is aggressive will bite the face, leash climb going for the face, multiple bite the body, causing severe trauma . Most "trained bite dogs" are trained with a prey drive by agitation and then rewarded by a bite..then further trained to bite and release on command. In a perverted sort of way your gal has been conditioned not by agitation , but, by encouragement of barking at doors or acting aggressive and then "put away". It won't get better unless something is done now. She is already 5 years old and obviously the behavior has existed for awhile.. I suggest rather then put the dog away, hopefully she has been obedience trained, put on a lead, and severely reprimanded for the behavior when it appears. She might have to be "set up" with someone knocking on the door and leash corrections used, with voice.
As a former private company security dog trainer , police/military sentry dog trainer and general problem solving with aggressive dogs is my opinion. My scars are numerous and mostly due to my carelessness and except for one incident with a young security dog, my bites were all committed by "untrained" dogs that were not taught to bite, including many retrievers. PM me if your interested in more. After reading my post I had assumed health issues had been ruled out as others pointed out, if not and before behavior is considered have a blood panel done.