It appears you can actually uncouple the ICE 3. For the longest time I thought it was not implemented, but that shows how wrong I was. All you need to do is turning the reverser off and setting the brake handle to SB (Schnellbremsung), then, as you probably tried dozens of times, press the decouple button (also working with Ctrl + Shift + C). I hope it helps and makes a few people happy to know it's possible at all.
