Run the game with mod installed once. It should generate kubejs folder in your minecraft directory with example scripts. startup scripts only get run once when the game loads. Those are for registering items, setting up configs etc. Server/data scripts are loaded when you join a world, those are better for things like clearlag, displaying custom login messages and world/player related things in general.
There's examples and pre-made scripts here.
Yes, use /reload. Note: Not everything is reloadable. Some things require you to restart game, some only world, some work on fly.
You can report issues here.
If wiki didn't have the answer for what you were looking for, you can join the Discord server and ask for help on #kubejs-and-code channel!