Commuted for years and carry more than you. Never found any issue wearing a backpack. I do suggest you pick one with two fastening clips one across the stomach and one chest. Means the weight isnt on your shoulders.
It has a frame with a mesh back to allow airflow under it so it isn't too warm and sweat-inducing except for the occasional baking hot summer day, when it can be. In the winter, it actually helps to keep me warm. I feel relatively comfortable wearing it, but it is nice to use panniers on the one bike I own which has a rack.
I definitely wouldn't want to carry my 2+ kg D-lock far in that pack!
Depends how far you intend to ride with your back pack. A small back pack is fine for short rides if the weight is kept relatively light. I used one for years for mountain biking with a 2 litre water bladder. So weight around 3kg all in.
Depends on the ride, for short rides if I'm not stopping I'll just get wet, for longer rides I use a frame bag - Restrap Large one, and I keep food arm warmers etc in there. Best solution is to dress for the conditions.
Personally I wouldn't be carrying things like arm warmers, I either wear them or don't. If I get hot I careful roll/fold them and put them in a jersey pocket. If I need a rain jacket this is rolled up and packed in a small drawstring bag, fits in a pocket.
I carry the following:
Under seat bag containing two tubes, multi-tool, cafe lock, £20.
Jersey pockets. Right side energy bars and meds. Centre pocket phone and wallet. Left pocket tyre lever, thermal blanket and rain jacket when necessary.
I don't carry heavy stuff like a D-lock as I never leave my bike out of sight.
Me personally. I have an old Deuter Race EXP AIR - Its got a fancy wire & mesh skeleton/frame system that holds the backpack off my back so air can pass through.
It has a few downsides though.... It costs £85 directly from Deuter but you can find them upto £15-20 cheaper elsewhere if you are really serious about one. Mines the 2012 version so storage capacity is 12+3 (litres) the newer ones are just 14L capacity.