Some of the certification requirements posted so far do not give the full picture. For example the safety factor of 1.5 does apply to static loads, however landing gears are typically safe life structure designed for a fatigue life that matches the aircraft service life. Therefore most landing gear components are stronger than they need to be for static reason. On the other hand you cannot use the safety factor to scale impact speed, at certain rate the shock absorber will bottom and the loads will rise significantly, so you can not assume that a landing gear can always take 1.5 the design descend speed.
It is also a requrement that in a minor (otherwise survivable) crash landing the landing gear must safely break off the airframe without rupturing any fuel tank, taking into account all reasonable combinations of horizontal and vertical forces. It does therefore not matter whether Douglas or McDonnell was designing the landing gear, the requirements are the same. Typically specific shear pins are designed to break off first, they are often tailored to exact rupture loads by taking material samples and machining the inner diameter to the exact required dimension.
Regarding the break off of the left wing, as a huge ammount of fuel is obviously released (and ignited), the wing must have ruptured in the area of a fuel tank that still contains a lot of fuel during landing. I am not aware of the exact fuel system layout. The left landing gear is still attached to the wing, so it looks like the wing fractured somewhere between the fuselage and the landing gear.
What puzzles me at the moment is the fully deployed right wing (Nr. 1) thrust reverser. At which point during the event was the T/R activated ? It obviously had time enough to fully deploy, which for a translating cowl design driven by spindles should take a little time, and not happen accidentially. Auto T/R at the firm first tochdown ?
Regarding the positive statements about FedEx, I work with some of their maintenance engineers and I am always impressed by their competence and their focus on safety. My thoughts are with the colleagues they lost.