You want her to stay in the sit until you tell her it is ok to move from there, so I guess in a way you are enforcing a stay also. Either sending her to her special place or keeping her by you will both work well in this situation.
The relaxation technique is especially handy for anxious dogs because you are telling them that there is nothing to worry about in the particular situation they are in.
She will definately pick up on your reactions to what is going on. So if you don't want her to bark/growl, try to act as normal as possible.
She sounds very smart and I don't think you will have any trouble getting her to be a very effective (when you want) guard dog
Kate