the fitters have connected to a wire to force it to be fully open constantly.
The only reason I'd do that is if it didn't stay open by itself. Even if it seems to, once it gets hot enough it might close. I would not use that fireplace with the wire removed, until I've verified that the flue is cooperative.
Of course mortar is heat resistant.
All the properties, cement, lime, sand, etc, are naturally heat resistant. However, mixing and applying the mortar can be enhanced by following a few extra steps. See link: