Believe it or not MT and I called SLR up the day we got our shackles to ask about that, because it didn't make sense. The thing you're forgetting (about the height lifting) is exactly what I forgot as well...it's not a perfect triangle.
Look at where the leaf springs anchor in the front, it's actually about 2-3 inches down from the bottom of the frame rails (which is why that area is so scraped up on all of our trucks.) When you extend the shackles, you're not extending one side of a triangle, you're extending one side of a (rough) parallelogram. You're right, the total lift is less than the 2", but I seem to recall the difference eye-to-eye of the shackles is about 2.5", netting you almost 2" before everything starts to sag.
Also, about the shock length...remember the stock shocks restrict your travel as well, if you completely droop one side you won't be at the bumpstop on the other side before the stock shock runs out of travel. I don't entirely follow your argument about the shock angles--even if you exagerate the angles you're still only halfway up the length of the leaf spring, right? So you would still need to extend the shock lengths unless the front shock anchored at the front springpack...
If I didn't understand what you meant, let me know...or if what I said makes no sense I'll try and edit it into some semblance of coherence.