
Hi again,
There are several more experienced on here than me in the behavioural department :P and hopefully some will be along later with more in depth advice covering all the situations you've referred to. Have you been to training classes with him to help socialise him as much as anything else? Also try and get hold of some of Gwen Bailey's books, The Perfect Puppy is often recommended on here but she does do several ones which may help- try this link
http://www.dogbehaviour.com/gwenbailey.htm You've certainly got the right idea already about not reinforcing his nervousness - perhaps you could try a distraction technique with him, even just a simple command followed by a treat (but not a treat on it's own or he'll think the reward is for barking :) )
Re. the layout of your front room with glass panel - I'd make it a room to the back of the house (kitchen for eg?) so that he isn't able to watch anyone approaching ;) I did this with one of mine at around the same age as your pup and although it took about 4 months (we don't get that many trade callers etc - mainly people our dogs know) it worked eventually. If your dog has a favourite toy, try leaving that somewhere within easy reach for you (not him!) and if he goes to pre-empt someone's arrival before you've spotted it distract him with the toy as you put him away - he can't bark with his mouth full :P Good luck, Teri