My shoulders are 46cm across, my hips are 37cm across, and the spread of my track pedals is 32cm. So what possible benefit would 20cm bars be in terms of squeezing through narrower gaps?

I do every day. most car mirrors never worry me. SUV and minivan mirrors are at chest level, Full sized vans at shoulder level, and trucks head level. Those are the ones that stick out enough to worry me, and so far the only part of me or my bike that has ever caught on a mirror is my bag.

i run risers without bar-ends, and i learned to be able to climb with them relatively quickly. its a bit trickier, but not impossible