Bars - sounds like you know what's comfortable, you just want to know if there's a question of control with drop bars and loaded panniers, correct? I have this combination for my similar-length commute, and control has never been a problem.

Gears - IGH is pretty low maintenance. I'd go with whatever will result in the most riding - if you're happy with SS, no problem, but if you think you'd ever bail on riding because you didn't feel like grinding or spinning that day, then go with IGH. As far as rear punctures, I'd go for tires with high puncture resistance, even if they're heavy - not a big deal when it's flat. Maybe have a rescue plan - I call my wife! It's only happened twice in 3 years. At that rate, it wouldn't be a big deal if the rescue plan was a taxi ride.

Panniers - I use Lone Peak panniers and they're very easy to mount and dismount. My walk is short enough just to carry them by the handle, though. Maybe if you're taking both panniers with you then you could have a shoulder strap permanently tied between their handles?