As a working dog, he needs a job. I think he is bored and assigned himself the job of "protecting" you since there does not seem to be anything else for him to do. (And I agree, this not really protecting your or his territory, this is resource guarding). Walks are good, but he needs to really be able to burn off energy. Is there some place where he can go and really run? Would there be some activities and games you could think up that you or your wife could handle, that would tire him out enough so that he is panting heavily, and ready to just plop down and sleep? Play crazy-all-out-fetch for a good half hour? Swimming?
In addition to physical exercise, I think he is telling you that he needs a job, and he needs to have his brain exercised and challenged. Perhaps you can take him tracking? have you done any training with him, would there be formal activities that will engage his brain that you can pursue? Nose-work classes are gaining popularity, and though I am not very familiar with them, what I have seen on videos so far really seems to make the dog work hard, but not the handler.
Maybe just put some time aside each day to work on fun obedience and teach him some tricks - those type of activities are fun for both people and dog, and helps to develop your bond with your dog.
I would not allow any resource/territorial guarding from him that you do not want. Since his behaviour is pretty predictable, mix up his routine and bring in new expectations for him, and be consistent on that.
If his obedience commands are solid, take it a step further. A sit means sit, and sit until you release him, even if he has to sit for half an hour. (of course, you would start with shorter periods.
) Put down a mat in your office, and teach him a Place command. He goes to the mat and lies down and stays there until released. Since he acts up when your wife comes in the office, a good way to stop the behaviour you don't want is to get him to perform a behaviour you DO want, like going to his place and waiting for a release.
But mostly, see if you can come up with some more ways to exercise his mind and body, in addition to introducing new behavioural expectations from him, that you would train and enforce as you would obedience exercises.