One more doorway protocol, but this one is for service dogs!
The first two options for showing your dog what to do when visitors came over culminated with the dog getting to greet and interact with the visitors -- the ultimate reward for polite manners and waiting.
For service dogs in training, it's usually best to practice having them *not* greet the person. Service dogs do not always get to greet the people.
If you have a service dog in training, it is really best if someone else can answer the door while you keep the dog leashed and with you, reinforcing check-ins and calmness. If you do not have anyone else to open the door for you, then just unlock it for your visitor, say "come in!" and move the dog away from the door (to whatever distance it's currently working at, so that it's most likely to be successful.) It's a good training practice to have your service dog in training on-leash during the entire visit.