"Heat soak" occurs whether the motor is operating or not because not every part of the engine operates at the same temperature and heat will tend to migrate to cooler areas, which, when the motor is running are areas surrounding coolant passages. "Heat soak is just migration of heat, nothing more. The heat will not be carried away as quickly when the motor is shut off, as the motor then becomes "air cooled" in a different sense. While it is true these same areas of the motor could become hotter than they had been when the engine was running, others are becoming cooler in order for this to happen, because no new heat is being produced since no more fuel is burning and the condition can be only temporary. Clearly this is a "basic" in engine design, occurs in all internal combustion engines, and, given proper design (which I assume with Mercedes) should be of no "dangerous" consequence and nothing to worry about. Should, for example, a way be devised to continue liquid coolant circulation to a fan-cooled radiator after engine shutdown, I would expect only a small increase in engine or component life not justified by the cost and complication of doing this.
Watter Wetter, which I use, is a "can't hurt" product, i.e., it is not needed since plenty of reserve exists in a good engine design. It may add slightly to this reserve. But, since the initial post to this thread dealt with the notion of (dangerous) heat in the engine following shutdown, when the coolant and Water Wetter (when present) no longer circulate, I suggest the effect of Water Wetter is miniscule in this situation.
Here is a truism of automotive design: reliability improvements possible by changes/additions using aftermarket products beyond those recommended by the manufacturer are 95% (if not more) mental, i.e., imagined. The designer sweated the small stuff and if you want to continue the process it's because it's fun and represents "enthusiasm" as opposed to real physical change. Maintenance remains essential, of course, and doing it "by the book" is both critical and at the same time all that is needed.
'88 300E (wife's)