How would you recommend building and encouraging that behaviour to take your lead with thresholds, i.e dog outside out back, person in home, or barks at roommate randomly from outside while im inside in lounge and roommate goes pastHow old is Shadow? This does sound like it could be protectiveness, but over-reacted a bit. Hard to tell without actually seeing her body language.
I would not accept this behaviour as "normal" or excuse it as being protective (I know you are only asking for more information, not making excuses, and I'm not saying you are [making excuses] - just saying what I would do if it was my dog).
In Schutzhund, before a dog can trial for a SchH title, they have to pass a BH, wich is a basic test of Obedience and temperament. Each judge will run their BH a bit different, but there are items where you have to walk your dog through crowds, stop and have people approach you and shake hands, and so on, with no reaction from your dog. When I did the BH with Gryffon, the judge instructed the "crowd" to act like they were having a drunken arguement. I had to approach the crowd, put my dog in a down, and he instructed one of the "drunks" to turn and talk to me and pat me on the shoulder. Dog was to follow my cue and not react.
So I would say that reacting to someone just normally approaching your son is not protectiveness from a place of confidence (unless the dog sensed something "off"), but more like a bit of bravado, a bit of "just in case". And if Shadow is that worried, she may be walking around with a touch of fear. Though it sounds like continued training and socialization will help her past that.
Just my thoughts on it, nothing definitive, others may have more insight into the situation. I know my rescue in general was very friendly with strangers when I adopted her, but there were a lot of growling incidents too when people appeared in places she did not expect to see them. It was simply a case of having her socialized more, and boosting her over-all confidence. The last thing she growled at inappropriatly is "ice-people" sculptures some of my neighbours put up along the road, as a lark. Gryffon ignored them, I laughed at them, Keeta approached hackling and growling. Once she sniffed and saw it was nothing, she relaxed. But this was the first time in years she growled at something that she was unsure about.
Like, both me and roomate have a fake chat then go out and settle him? He loves my roomate, but sometimes does what you're describing with the door as a threshold