Yes, I certainly agree it's puzzling!
But it does work. Once the folder C:\Program Files\Google\Google Earth\res\shapes has been changed, following the rules described up-thread, the new icons (like my beer mug) can be used consistently, surviving GE sessions and PC reboots.
However, the KML appears to contradict it, as you say. So I'm very curious to understand exactly how it works!
BTW, I posted my original query simultaneously here and in the GE forum in this thread:
I had a prompt reply saying that although this had been a long-standing request, unfortunately it still wasn't possible. Following SpiderX22's revelation that it was, I duly reported that, with a link to here. So I'm surprised not to have heard back from the GE Help Forum experts, either there or here.
Terry, East Grinstead, UK