Hey Vometia,
I strongly believe the chest you mentioned on Sentinel Island is also accessible after you take over the Battle Tower ! You can fly as a dragon and don't be afraid of landing on all possible places that are "higher" walking on Sentinel Island in humanform earlier on !

If a place is ful of poisonous gaz I always fast run/jump and then immediately transform back into dragonform .

Good luck & enjoy your DKS time !

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !