I've used DAP diffusers and DAP collars and to tell the truth I am not sure they help. There was no dramatic change with just DAP. All of the times we have a stress problem I am doing corrective training and calming training so the situation does resolve with the right training. I've come to the conclusion that DAP doesn't hurt but it is very expensive. You can find it much cheaper on line and buy refills so that you can use the aerosol more than once. With the collar it's simpler and as long as your dog's skin doesn't react go ahead and try.
What I have heard does work better is a little rescue remedy added to water. Again I'm skeptical but if it's harmless why not. Training to desensitize and calm are always the best answers. Some people talk about a stress wrap which is basically the same as putting a tight tee-shirt on the dog. The behaviorist I worked with to help our scared girl said the wrap did work but was much more expensive than a child's tight tee-shirt would be. It should be snug around the body but not tight or constrictive. Remember when dogs lie down their elbow joints get bigger so to avoid blood flow constriction you might cut out the sleeves if needed.
Two great books are Click to Calm by Emma Parsons and Control Unleashed by Leslie McDevitt. These involve distracting and redirecting the dog and rewarding for calm behavior. McDevitt's book is a little more involved but very thorough in discussing method and reason behind the training. Good luck...I know it's not easy.
What I did with our stress barker is distract her any way I could, squeaky toy, stand in front of her, block her and give a quiet signal and click treat any moments of silence no matter how brief. As we progressed and she barked at elk for example (we moved so elk are new) I would wait for her to inhale and click. Gradually the quiet got longer and I clicked for quiet looking, quiet look back to me, any relaxed behavior like sit or down, I would yawn at her and look sleepy and click when her ears relaxed, click when she blinked. This is covered in both books.
What finally happened is I could do it from further and further away and she would come get the treat. Then we would do mini-obedience sessions with clicker and treat and be very fun and positive. We did it long enough that she forgot the thing that she was going to bark at. If I stopped and she tried to fly right back outside it wasn't long enough and we did a little more. Then if I clicked just before she barked-when she was still relaxed but getting ready to tense she would come off and get a treat. The goal was to make the stimulus for barking the stimulus to come get a treat. Now we do it outside with the elk there and most of the time now she ignores them.
for random barking inside as a sign of stress i would def leave a frozen kong to distract her when you leave. practice coming and going but leave just for a few seconds and come right back. do all the things you normally do like get keys, coat, etc and do leave/come back for very short times. make them a little random so she can't predict it. then after a few short ones add a 5 minute one in, 10 seconds, 7 minutes, 30 seconds. the point is to teach that you leaving isn't nec always for a long time.
try to make coming and going very boring and uneventful so that she's not excited when you come or go. the less emotion attached to this activity the less stress the dog should feel.