Seems you sound so frustrated over those keys, I give you some answers to chill you down, since I didnt actually touch any chests in maxos temple yet even though I am already over sentinel island
1. Key on the so call big lantern. That key just open the locked chest before the spiral staircase. The chest can be open with high lock pick skill as well

2. Key in amdusias's room is for opening the chest which contains orbis arcesso part 1. Where as key in fendrel's room, its for the chest in his room, which possible to be unlock with higher lock pick skill as well

I think there is an older thread with regarding keys in quest tab. I already mentioned before that there are quite a number of chests which had keys for them, but still able to be unlocked with lock pick skill. In the end, if you found the keys only after you had unlocked those chests, you will be scratching head like you are now
PS: I just patched the game with the latest 1.03 patch, but still not able to get that half buried chest open.