Ok this is going to be long winded also,

Yes Dobes make great watchdogs, so do Poodles no joke. Dobes will and do like all dogs read your body language, you stated you put a deathgrip on his collar and leash when passing, I reccommend you dont do that instead put him in a sit and let them pass, reward him for his gentlemanly behaviour.
Also teaching him to sit when you answer the door and to stay sitting will defuse his lunge response. You didn't state if he growls or bares his teeth or is showing true aggression or just a response that hasn't been redirected. I raised rotties for 23 yrs and my parents raised danes, gramma had dobes, so I am only speaking from our experience, I had a rottie that did the same thing (female) and never realized that I was feeding the response by reacting to it, these breeds need a firm hand and a no nonsense attitude as to who is alpha, and what is expected of them. Try not to tense when he does, put him in a sit and retrain him to ignore the stimulation, it wont take long and he will be a perfect gentleman, they desire to please. As for not knowing if they have a treat or a gun? the more socialization a dog has meeting 9999 great ppl gives them the ability to know when that 1 is "off' somehow, because dobes have a natural guarding instinct your job as his alpha is to not encourage it, if it was ever necessary to protect you from an actual threat he will without hesitation and without any additional training.