speaker break-in, how loud?

NO, you speakers wil not suffer negitive effects from underdriving them. Only clipping from your amp or overdriving them will damage them.

NO, just play them at the volume level you would generally listen to music at. B&W recommends 15 hours of break-in, which means just playing them for 15 hours. However the Kevlar driver, IMO needs more than 15 hours. They will sound better at 15 from the box, & improve at 50/200/500 hours levels. As a B&W junkie for the last 12 years, I still have a pair that old that still sounds fantastic! I generally do not listen to them to critically evalute them till after 200 hours. Enjoy!

Surely you shouldn't drive your speakers to clipping.
I'd say to stay aware of initial "critical testing" for 50...60 hours.

What you are breaking-in is copper,in the binding posts,internal wire,drivers,crossover parts. This normally takes about 330 hrs. of music signal played at normal listening levels.

Oh c'mon. Copper is hardly the only part breaking in with new speakers. Consider the rubber driver surrounds, drivers themselves, and driver - voice coil assembly for starters. There is more to audio than metallurgy.