I can't answer your first question because it's unclear to me where you are talking about placing the steel panel - what do you mean by "behind a fireplace wall" - in another room? As far as your second question, yes there is a code requirment for how close any combustible material can be to a firebox opening. This varies depending on your local building code but is typically 12" (sometimes less, but never as little as 4-6") - check out your local code at your city hall.

Most fireplaces also have specifics on clearances so its a very hard question to answer, in fact I don't even know if I understand the scenario.