Medium size cargo solution for road-bike for commuting
I would really recommend re-looking at a backpack.  There are a large variety of packs out there and the drybag styles ones seem to get better every year.  I have found that a backpack is a more dynamic way to carry weight.  You might look for one with better airflow built into the padding section.  I find that if I dress just a little more lightly and have a pack with good airflow, "sweaty back" is a bit less of a problem.

If you can't find a drybag style pack that works for you, you could always look for a base pack that fits all your requirements and then just stuff a uber-light drybag in it.

Along those lines, you might also look at Revelate Designs new holster bag.  Essentially it sits under your seat and provides a secure place to stuff a drysack with ease of on/off for the drysack.  Your personal opinion will dictate whether it meets requirement 6 or not.