I'm not sure what's going on with the first but, but I know you can change event arenas.
In the Events and Events_2Player documents you can find the settings for each event. The last 2 lines of text for each event define the arena and it's preview image. Find whatever arena you want from the events (things may be more complex for arenas that aren't in events) and copy/paste them to other events.
Example of those 2 last lines I mentioned:
