I'd say that an automatic is a must since most of the day is spent in the car.
If I drove from W.London to Central London, then N.London, C.London, S.London, E.London, C.London, S.London, W.London during a week day that would be about 60 miles a day, or 300 miles a week, or 1,200 miles a month, which is still less than the 15k that your wife is doing. Doing that journey everyday, would put me in the car for about 4-5 hours each day.
I don't know if the town/city which your wife is going to be driving in is quite as congested as London, but I'd say:-
1. An automatic is a MUST.
2. The steering (and clutch, if relevant) must be feather light. Handling and steering 'feel' is a non-issue. In fact, the heavier the steering / clutch is, or the more feel it provides, the more tiring is going to be with the type of driving.
3. The car should have good visibility.
4. The seats should be reasonably comfortable.
5. Good air conditioning / pollen / particulate filtration is must. There's NOTHING worse than sitting on say, the A40 out of London in non-moving traffic in the middle of summer without air conditioning (I've had the unfortunate experience of that on too many occasions!), not least because of the heat (tiring) and the amount of particulates (soot!) you breathe in from lorries/vans/other diesel cars.
6. Avoid low profile tyres / big wheel combinations that result in a jarring ride over ridges, potholes etc (VAG cars seem to be particularly susceptible to tyre/wheel combinations).
I hear what you say about the Micra, but seriously, try a high spec petrol auto for a couple of days and see what you think - I think it nails the criteria (including the ones I've listed above). Cheap insurance too. My parents bought one of these last year and although when I first drove it, I didn't like it, it's really grown on me. For town driving and crawling through stop/start London traffic all day, I prefer it to my manual 323i. It's also a very disarming car - people let you out of junctions (which they NEVER do when I'm in my BMW) and you can squirt through gaps, because other drivers don't expect it from a 'bubble' shaped car. Less road rage from other drivers too. The reactions I get from people when I'm driving the Micra compared to when I'm driving my BMW is like night and day. For this type of driving, the traditional criteria as to what makes a 'good' car (eg. ride/handling, power/weight ratio, looks/styling, nice interior etc) really does change, almost completely.
The Fiesta is a good car too, and like the Micra, you get a lot of (late reg) car for the money. Not sure if you can get a TDCi with an automatic gearbox though.
[Edit: Just seen your edit. Oh well, at least I tried!]
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