What makes you assume Roxas is on Destiny Island?aqua's crying in dark meridian.. u can see the moon.. while ven wrapped in white cloth is in destiny island..
aqua sitting beside ven?? i doubt.. u can't see blue hair there.. it's most likely the kingdom key..
we don't know about his keyblade getting wrecked..
And as for the Keyblade. In the KH2FM+ secret ending, it is shown that his Keyblade gets basically snapped off the handle after he's frozen and thrown off the mountain.