captchunder:
I always thought that 3 scenarios must be check out. One is when there is an engine failure and then is there the driftdown to max EO alt. and then to ETOPS alternate. Second, as you pointed out is and engine failure with decompression and from there to an alternate at 10.000'. The third, (which is always the most limiting fuel wise for us any way,) is just decompression and from there to an alternate at 10.000'.
For a 757 from an island in the Atlantic to the East Coast of the USA, it is worse fuel wise to lose pressure than it is to lose an engine and pressure.
Best regards