I certainly do not know all the answers, however,
Nothing you put in the FMC will make any difference. On the descent page there is provision for anti-ice on and off levels but as the VOL 2 states it does not affect the a/c performance.
Most jet jockeys that have been around for a while will tell you that having "gates" on your descent is the way to go. I.E. an ALT at a given distance. It is simple mental gymnastics that provide a simple back up to what the FMC is doing. NEVER EVER have total blind faith in the FMC!
The FMC always flies a 3 degree profile in path and will generally hit profile limits pretty close to the mark. You have probably found that the heavier the a/c the more difficult it is to hold speed when descending in path. Some pilots swear by speed at all times and forget about the path (as long as they are above or below any limiting steps). I always put 500 feet below a AT or BELOW step on a STAR. This will account for any speed or path excursions that you have to fix at short notice, ATC restrictions etc.
Dont be affraid to use the airbrake at the higher levels if the speed starts to get away from you. Disregard the "he man" B.S. some testosterone heads carry on with about using the airbrake. It is there for a reason. I have found that overspeed disconnect is generally only a problem at higher levels. I never descend early for no other reason than economy, although that can be argued against as the saving is minimal. I rarely have disconnect problems any more due to a bit of experimentation, and as I said they all started at higher levels (above 30,000 feet).
Good luck.