Isn't that gasoline engines that have a narrow range of near-peak efficiency? Diesels do not use airflow-restriction to throttle power. Gasoline engines (current embodiments, at least) do use airflow-restriction to throttle power.
Nope. Read the wikipedia article, especially the last paragraph of "Reliability". (Part of what's happening is that diesels have a smaller overall range, so even if the "90% peak" ratio is bigger, the width of the range is smaller.)
Also, diesel engines' fuel consumption doesn't vary as much with load as do gasoline engines'. (Again, see the wikipedia article.) This makes diesels especially good for hybrid use as it means that they can drive the generator "for free" while the car is being driven with the diesel running at less than full power.
For smog, especially particulates, that depends on what you mean by "some time". Until a couple of years ago, diesel cars were't close to satisfying CA's requirements. (The switch to low sulphur may have also helped.)
FWIW, diesel-hybrids would be especially effective because diesels have a much narrower "90% of peak efficiency" range of RPM and load.