I have some questions about spark plug gap. My 1981 320i USA 49-state model uses the Bosch WR9DS plugs. The gap is specified as .024" + .004".

a. Does this mean the optimum gap should be .024" but that I can increase it to .028"?

This is really a range -- new plugs gapped to .024" will still work well when the gap grows out to .028" (maybe a lot more, but that's how far the factory is willing to go with their recommendations.

b. If I can increase the gap to .028", what are the benefits or why should I do it?

In general to us old hot-rodders from 40 years ago, a wider gap meant better starting but some risk of high-rpm missing, whereas a narrow gap meant better high-rpm and full throttle power, but sometimes harder starting. In those days, we tended to keep the gaps narrow because it was the top end on a warm engine that we cared about.

c. A reader a few weeks ago suggested increasing the gap even more, to about .032". Why?

Maybe for better starting and low-rpm smoothness, but a lot of old-time (like mine) thinking doesn't apply to electronic systems.

d. My plugs foul with oil during stop-and-go driving. I realize that eventually the car needs major repair, but will a wide or narrow gap help reduce the fouling?

A narrower gap might help the spark continue to jump when the plug is slightly fouled, but what you need to reduce fouling is a plug type which is one or two heat-ranges hotter in its heat transfer rate (and then be careful not to overdo the full-throttle running in a way which might cause preignition.).
Thanks!