Commute, panniers, snow and ice.

A Carradice bag, something like a Nelson long flap. I found that keeping the load central is a great help. You probably know that Carradice have a good selection of cavernous saddlebags, when my bike was this commuter's transport I didn't think they could be bettered.

Personally I just use one every day with some tools, batteries, waterproofs and my lunch and never feel it is upsetting the handling. The only problem is I put it on the right so the reflective panel is nearest to the drivers passing me but my sidestand is on the left so sometimes the wind has an easy job blowing the bike over.

I'd say two, but don't add ballast just for the sake of it. An overloaded bike is a lot worse than an unevenly loaded bike in difficult conditions IMO; you have slower handling, lighter steering and other such miseries to contend with.

I'd be tempted to split the load between two, so you have a nice low centre of mass in partially-loaded panniers and don't get that weird swaying movement that comes with a single pannier. In my experience, that gets exacerbated in slippery conditions.

Well first of all, the answer to the question inthe other thread must be a resounding yes if you are only cycling three miles to the office with assist and still being hot and sweaty. 

I personally would just keep to one as that is what you are used to with the bike and making changes to I assume balance it out, could alter your feel. However carrying extra clothing/kit for emergency needs, (walking and pushing the bike due to weather/ incident will cause you to be colder then cycling) is definitely a consideration if you have the room/ extra panniers.

BTW, have you considered taking several sets of clothes in the office at the start of the week and then only bringing home the worn clothes each day? You obviously need the space in the office but it is something I do.