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 !
