If the challenger has gone ahead and built a boat before knowing the rules, then that is their bad luck. They'll need to start grinding carbon soon.
90ft x 90ft sounds simple. Now if you can change how that is to be applied at the last hour who would ever offer a challenge. A minimum time period between finalising contest rules and start of competition should be prerequisite in the rules, but apparently not. With the amount of legal wrangling already completed you would have thought agreement could have been reached ages ago. This is not yacht racing this is just power play. If it was held under ISAF juristiction surely charges of unsportsmanlike conduct and bringing the sport into disrepute would be appropriate.
Cheshirecatman