OPEN DES:
Your choice of names speaks volumes about your prefered descent mode!
Jokes apart,
1. Managed descent does fly a geometric path between constraints, but what you are not saying, is that if necessary it will fly it at idle thrust same as open descent.
The path of the managed descent is calculated:
DES MODE
The DES mode guides the aircraft along the descent path computed by the FMGS.
The system computes this flight path backwards from the deceleration point up to the top of descent (T/D) with respect to the speed and altitude constraints .
Internally, the computer divides the descent path into various segments, depending on the relative positions of the constraints. It starts at top of descent (T/D) by setting up an "idle" segment that takes the aircraft down to the first constraint, and follows this with "geometric" segments between constraints.
DESCENT SPEED PROFILE
The descent speed profile is usually the ECON speed profile, amended by any speed constraints and speed limit contained in the flight plan.
Before the descent phase is active, if the pilot does not intend to fly the ECON speed/Mach profile, a different speed or Mach can be entered to amend the speed profile :
The pilot may enter a Mach number and/or a speed in the MANAGED field of the PERF DES page (3L key).
2. Idle for descent will be the same in DES or OP DES, the idle will increse for anti ice regardless of descent mode
Also on the last paragraph, you are contradicting yourself, if you select cost index of 0 your speed for the descent will be slower(not green dot, but closer to 250KTS on the A320) which means you will spend less time at altitude, that is really where you want to be for fuel savings.
I don't know what your airline's sop's are, but Airbus procedures call for managed DES to have a priority over selected DES whenever possible, for economy reasons and another big factor: constraint compliance.
Here is some more info:
FLIGHT OPTIMIZATION
The optimization function computes :
The best target speed for climb, cruise, and descent (ECON SPD/MACH).
The best descent path from the cruise flight level to the destination airfield.
An optimum flight level (for pilot's information).
BEST TARGET SPEED
It is computed by the FMGS (except for V2). The best target speed (ECON SPD/MACH) is the basis for the managed speed profile.
ECON SPD/MACH is a function of :
Cost index (CI)
Cruise flight level (CRZ FL)
Gross weight
Wind and temperature models
The computer processes the ECON SPDs for the climb and descent phases before the initiation of the flight phase, and freezes the values once the flight phase becomes active.
G