Carl:
Not sure where Zama is or what the weather is like during the winter... We lived in a little town on the Tama River just outside of Tokyo next to Kawasaki.... It had a pond similar in size but not as good looking, no water fountains but a small water fall fed by a pump in the pond...
Although we had snow three times while we lived there, it usually melted off in a day, never lasted more that two days on the ground, and the pond never froze over, not even a little ice on the surface... There were scrub koi in the pond which we never saw until the residents would feed them bread, rolls etc. from a platform at the edge of the pond...
Seems to me that your only problem would be the trout tournament...would think that if you feed floating pellets, the koi would be the first to take an anglers line...especially if they use corn and/or peas like I've seen at some trout fishing ponds...
Looks like you're on your way to a very pleasent stay...
Aloha! Mike T