Turbulence can be a little uncomfortable and tedious but is not in itself dangerous unless there's windshear also and you're at lower speeds such as during TO and landing, and I've never heard of a plane crashing due to turbulence alone.
What you probably would consider to be bad turbulence, we as pilots probably would normally consider to be just light to moderate. In 11 years of commercial flying I can only remember 2 or 3 occasions when I've experienced genuinely severe turbulence.
As to how much turbulence an aircraft can take before it suffers damage, I think you'd be surprised. Videos of wing strength testing, show they can almost be bent vertically before breaking and I would be very surprised if turbulence could damage a modern aircraft.
You also asked about why the pilot didn't change altitude, and there may be various reasons. Perhaps other aircraft reported the turb to be the same or worse above and below, or maybe as is often the case over the atlantic he was unable to get clearance. Whatever the reason I'm sure it would have been considered.
Also air pockets don't really exist, they are just turbulence pobably caused by high level winds.
Anyway hope that helps, good luck with you're flying lessons hopefully they'll make you more comfortable about flying!