You aren't altogether incorrect, 346. If you do change the CI your optimum level will also change. CI=0 is equivalent to minimum fuel consumption (maximum range). CI=999 is equivalent to minimum time ("defined in FCOM 4.02.20 p16). However, even with an amendment to CI, the effects of wind, temperature and gross weight will still alter the OPT FL.
Further, if you refer to FCOM 4.02.20 p15, the "OPT FL is a compromise between fuel and time saving". What part of that sentence does not scream out "fuel efficiency"? That is why pilots are allowed to manually edit the CI, be it at the behest of the company or as part of a pilot-initiated strategy.
As for the "ballistic" flight path, I agree - that's exactly what I inferred earlier. But it never happens because of ATC. You said it yourself.
And yes, you can set up the FMGC for arrival on the descent, but I'd much rather be monitoring the descent!