[an1sotropy]

But which has a greater effect on removing heat from the cream-containing vessel: the decrease in temperature from the dissolution of salt, or the more efficient thermal coupling to the vessel provided by the salt/ice slurry (versus the original solid ice chunks)?

The goal is to remove heat from the cream faster than the system as a whole warms up due to room temperature. I thought the value of salt was to help the cream win that race by making a better heat sink.

[Karliss]

If the primary benefit of adding salt was improving thermal coupling through liquid by melting some of ice then you could achieve the same effect by adding some tap water. Which in my opinion would be a lot simpler and less messy than getting salt involved. Some energy would be lost to cool down tap water, but as mentioned in the article phase transition takes a lot more energy than changing temperature of water.

[an1sotropy]

If you had really cold ice cubes, already tightly packed, then the water you add would freeze, making a solid ice sheath around the cream-containing vessel, and yes, that would work great.

But with too much space around the ice cubes, or ice cubes that aren't cold enough, adding water will just give you more cold (but not freezing) water.

I think people have converged on adding salt to ice because it's so forgiving (for a variety of ice cube temperatures and geometries), and the salt itself doesn't appreciably heat anything (unlike your added water). Other comments here quantify this better than I can.