I replayed FFX for story
and do not worry folks.......
U will DEFINATELY use the destruction sphere and get the treasure..U will 100% use the sphere and the chest is located RIGHT infront of the exit to go pass the trials
as for the other treasure u missed....its juc a chest opened by the Bevelle Sphere....
so do not worry
if u got Bahamut....thats means u definately got the Bevelle Destruction Sphere treasure
juz that u didnt know and forgot u opened it