Look, OP, if you want a "road bike" style bike, but also want to do some light trails, you're going to need a cross bike. Many cross bikes come with mounts for a rack - which IMO is essential for bike commuting. A backpack kills your shoulders, shifts weight, and produces copious amounts of back sweat. It'll be worse with a messenger bag because it only has the one strap so it'll constantly be sliding back around your front, and moving around. Terrible idea IMO. Rack + pannier is the way to go. (or trunk bag)
Traditional road bikes don't usually have rackmounts. Mostly because they aren't built for utility.
If you want to do rougher trails, you're going to need a mountain bike. Even cross bikes aren't designed to really go bombing through the woods like a traditional trail ride.
I have a Surly Cross-Check. It's a do everything bike. It is a road bike, and it's a cross bike and it's a commuter. It has all the mounts for everything, it has space for narrow slick tires or wide knobby tires (I currently run 700x32 on mine, but I can go wider if I want. Some other cross bikes have this option as well. But you need to look.
I'd recommend it if you want to put tires for trails and still have a bike that looks like a "road bike."
I'd also recommend the All-City Space Horse (more expensive, but gorgeous to behold).
My wife has a Specialized TriCross, which is also very similar. Not sure how wide tires it can go up to, but it currently also has 700x32 on it.