Yeah, I totally agree about wanting to be the lone chest/coffer hunter. I hate doing it when there's competition, because it always seems they'll beat me to it. Especially in Oztroja, there's always high level thieves that don't worry about getting aggro in the lower coffer zone, so they just flee right on through. I, on the other hand, will get aggroed by anything in there. Oh, and if ever I do have the place to myself (GASP!), rest assured that the coffer will spawn in the leech pool ><. Then it's either give up, or pick it and know for sure I'll be using my 2-hour and warp scroll when those suckers aggro me. I seriously wish Thieves has an ability similar to Hide which works on sound detection. Actually, just yesterday I did have the castle all to myself for about 4 hours and the coffer never spawned in the pool! 3/5 coffers I got had rings in them too!! I seriously wouldn't recommend hunting Oztroja until you hit 55. I was doing it at 50 and got a LOT of mimics. Now that I've gone back at 55, I've gotten 1 mimic out of ~5 stacks of skeleton keys, though I still fail quite a bit before successfully picking the coffer.
But yes, Fei'Yin is a GREAT alternative when Oztroja becomes one huge thief sausage party. Dude, level your white mage or red mage, you will be SO glad you did. It comes in handy for much more than just chest/coffer hunting. If you do want to stick with the silent oils, you may consider leveling thief to 61. I was told that's the level where the shadows on the first floor will no longer aggro you, so you'd only need them when heading downstairs to check the northwest 3 spawn points. And get the Fei'Yin map!! Reference my post above about it, as the quest is #3 in a stack of quests for the Optistry in Windurst. The stairway leading into the basement of Fei'Yin is just southwest of the main room in the center. Once downstairs, keep sneak up at all times and watch where you cast as colossus, dromas and weapons there will aggro magic. The stairs leading back up to the 3 spawn points are northwest on the basement map.
I hope this helps you! Level that thief! Level that white mage! In time, all good things will be yours!
Main Job: 74 THF
Other Jobs: 42 RNG, 40 NIN, 38 WAR, 37 WHM, 30 COR, 18 BLM, 14 BST, 10 MNK
Bastok Rank 8, Zilart Mission 14, CoP Mission 5-2
Race: Hume Male
100 + 3 Alchemist
54 + 1 Goldsmithing