Is stem cap redundant after tightening stem bolts? Is it right to think that after the preload is set, and the stem bolts are tightened, the stem cap becomes redundant? Will the preload be held fixed by the stem? If so, does that mean the stem cap can be removed after preload is set?

Yes this is correct. Once the stem bolts are tight, they hold the system together. The top cap can be loosened without releasing the preload. You could try it out and you should find that the stem remains fixed and no play will develop.

You'll still need the top cap next time you want to preload the headset, so perhaps not redundant... maybe furloughed is a better word ;)