There should be no risk to your wood mantle, assuming it was installed per instructions, usually 1" clearance from flue and firebox. The fireplace box is usually well insulated, so the glass might be warmer, I'd check with a infrared thermometer. The blower might not make the fireplace "more efficient" but it will circulate that heat further from the fireplace. It will also help average the temperature so its not crazy hot by the fireplace and cold further away. You might consider a couple of computer fans near the sides, even running the ceiling fan backwards if you have one might be just as effective.
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