Smilin',
Are ABs routinely being flown too close to the coffin corner? If so, is it for reasons of cost?
No, they're not flown too close to "coffin corner", precisely because of the high cost in fuel flow. That said, one takes what altitude one can get on the ocean crossing. Taking a high altitude early protects one's position while taking one too low may not permit one to climb later, depending upon traffic. Westbound, er, no, eastb..n...yeah, eastbound out of Hong Kong/Beijing etc across Japan to N.A. can be a problem. I think out of Hong Kong with the 340's hair dryers we always had to cross ELATO (if I recall the name correctly) at 290 because of weight and even that was a stretch. Three hours into the flight however, reduces the weight and improves the MAX ALT and Optimum Alt substantially.
FWIW, I don't think, (never did) that "coffin corner" had anything to do with this accident. The airplane's flight planning package, (fuel flow, weights, temps) and digital presentations (the actual lowest and highest speeds available are presented right on the Primary Flight Display) and the cost of fuel all mitigate against climbing too early in the flight notwithstanding all the above - FL350 is not an extraordinary altitude for the 330 at that point in the flight - pretty normal stuff, really. The stall speed and the Mach buffet speed at that altitude would be, guessing, about 100 kts apart perhaps slightly less, say 190ks to 290kts IAS, (the stall charts in the AOM only go to FL200 and there is no Max Dive speed indicated but maintenance isn't even interested in overspeeds until they're 20kts above the VMO).