all your points are valid and you highlight most of what the Autotether unit is designed to prevent.
The unit is not linked specifically by radio only with a known range of 30' as you point out.
It works very simply and is activated immediately on immersion in water of a certain mass.
In other words if you fall in the unit immeditely cuts the engine or activates an MOB alarm. It does this by monitoring the radio signal and if this changes for any reason as caused by the water mass surrounding the FOB for instance it activates. If you move outside its range it also activates and so comes in handy for children around water and keeping an eye on them. It activates within 1ms.
The unit will not activate by simply getting wet, say on a very poor day, sea spray etc when out in rough conditions.
Interestingly, last year doing the Round Britain we had to have all crew on killcords with first man out kills the boat approach. Cables every where and lots of false alarms with people forgetting they were attached. The Autotether would be a simpler more cost effective solution in this case.
I have used the unit all year and having got into the habit of using it, now, I simply get on board, activate the unit and start the engines.