Might contain slight spoilers
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As far as your problem one is concerned, I'm not sure if I understand you correctly but I think this is what you need:
The key to the cell of Garren's child is on the body of Firkraags lackey (the wizard). Kill the mage and you will have the right key. The disappeared key that you are referring to must be another one (there are about 30 keys in the game you know)
As for the second problem, no, you are not "meant" to do anything specific in the tanner quest...when the game "means" you to do something, it usually forces you to by leaving no other options open (for instance in the beginning of the game you can only go to the Slums, so that you will meet Gaelan Bayle). In the tanner quest, you have two options: check up on the tanner yourself, or have Aegisfield the inspector do it. If you did the latter, then Father Bronze gave you a good clue as to what to do.
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Melusine, Archbabe of the OHF and the LH
Your voice is ambrosia