Atom, at risk of annoying many in this thread:
It appears to me that the point you are trying to make is that with a roughly level pitch attitude (as we roll down the runway gathering speed for takeoff), the vertical load is mostly supported by the pallet, and as attitude increases (rotation, and then climb attitude attained), a percentage of the vertical load (Frame of reference earth, not deck of plane), aka the load from gravity pulling on the mass of the stuff loaded on the pallets, is taken up by the restraints (varying as pitch varies) until climb is over and the plane returns to level flight ... how many degrees nose up that is being a variable with a small magnitude.
That said, and PJ2 making this point
The longitudinal g's on takeoff are in the order of 0.3 +/-, and on rotation the vertical g is similarly small, around 0.05 to 0.1g bearing in mind that for most aircraft types the accelerometer is placed near the center of the fuselage, (gear well area), although unless there's a recent change, the B777's is mounted in the cockpit.
I considered sines of pitch angles to show the magnitude of forces on the restraints, not on the pallets
I considered that a previous poster with some experience tells us that the restraints are typically able to handle 1.5 times the rated load,
and with a bit of back-of-the-napkin figuring arrived at 30 - 45 degres pitch up before you ran into that strap load capacity being put to the test.
This leads me to a catch-22 for offering strap failure on lift off,during initial climb, as a cause, since the limiting load would not be applied until the nose is well up there, but the nose wouldn't have gotten well up there unless someting failed and all that mass moved ...
I may have plugged in the wrong values, however.
With the previous mission being safely launched and landed ... I run into a dead end for the straps being the culprit.