Nothing needs to be noted in the code box. Sometimes the officer will write "R" for reduced, but definitely not mandatory.
School zone does not need to be added to the wording. You may want to check the signage in the area to make sure the school zone signs/flashing lights were clearly posted for the direction you were travelling.
In regards to the officer not being present at trial, sometimes they will provide their contact info to the prosecutor to call them when ready to proceed. Having officers sitting around in court all day isn't really that efficient. Often they are working nearby and will be ready to attend. Not that uncommon. You could always call their bluff and demand to proceed to trial, but why would they lie about that?
I don't think you're going to be a better deal than 15 km/h over. Plus, you run the risk of having speed amended back up at trial. In my opinion, it wouldn't be worth the time or effort.