[masraum]

Can you get to where the fireplace is connected to the wiring? From your description, you've got the other end of the wiring disconnected, the end away from the fireplace. If you can get to the end closest to the fireplace, then you can check the wiring between the fireplace and the junction box where all of the wiring for the outlets and fireplace feed from. That feels like the next step.
I think you currently have access to the "*" with one of the wires disconnected, in the diagram below. If you can get access to the "@" in the diagram below, then you can test the wiring between the * and @ and test the wiring of the FP. That would be my next step, to try to get that access.
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| box | *=============@ fireplace
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