I looked at your questions, and it seems to me there isn't any agreement on any of that stuff. The trailer question ought to be a mater of record, and around here I rarely see trailers. The big knock against them is that a lot of people own a bike and it has a rack, and they have some paniers, so the moment they think they might want to go touring they may already have what they need. If everyone already owned trailers for carrying groceries then it would seem they would be a natural choice for when they go touring. I have used both trailers and paniers and prefer paniers, for much the same reason I prefer throwing my gear in the back of the car, vs. carrying more and hitching up the trailer. If I needed to carry twice the gear I do for unsupported riding, say I had special gear or lots of water, then trailers would be my choice.
Also I use the same paniers and racks for touring and comuting, I just only use one bag for comuting, If I had a special comuting bag it might well be assymetric to maximize the utility of one bag. Or top mounted. Which would be logical, but not really supported on the average current bike.
I remove gear when shopping etc... around town, the comuting use. I leave gear on and simply don't stop where I fear theft while touring. I haven't faced the problem of say touring europe, where a major goal might be to see the cities, and not have my gear stolen.