Personally I commuted for many years, often in very dense traffic (including Central London before there were many cycle commuters on the road) in all weathers, and in the dark for almost half the year. So most of your objections cut no ice with me unless you feel you are too inexperienced to know how to ride defensively in traffic. If that's the problem, get some advice.
The only question I would ask is, what on earth are you carrying that requires a backpack weighing 15 lbs? A change of clothes weighs about 3 lbs. if shoes and a suit are the issue, next time you use the car, take some in and leave them at work. Cycling with a heavy backpack sucks.
Commuting by bike is a life-enhancing experience, and doing those 20 miles each day will bring you on in leaps and bounds not just in respect of your fitness, but also your confidence and enjoyment on the bike. Just do it.