Since all the facts of the 'investigation' of how and who started the fire are sealed in perpetuity in the juvenile court system, we have too little information to assign 'responsibility for the destruction'.

How many years was that facility sitting idle and unprotected before the Alliance paid for it in April of 2015?

"The Emeralds played their last game at Civic Stadium on Thursday, September 4, 2009,"
"On June 29, 2015, Civic Stadium was destroyed by fire.[23] Two days later, officials charged four pre-teen boys".  

So it was empty, day and night,  for almost six (6) years before the Alliance bought it.

Not knowing the ages of these four boys, one or more of them might not even have been born when the Emeralds left for the last time 6 years before the fire.