Ah, I wouldn't think so. Photo B is the external side of the chimney, which would have originally been outside the house (the external wall). So i'm guessing your living room was added onto the house at a later stage. So it was never designed to be a double sided fireplace, like some are. And yes, it is a structural part of the chimney. By the way, your bedroom fireplace is a beautiful original mantle, I wouldn't touch it.

I agree with me me, attach the stop to the door. Place it exactly where you need on the door. Close to the hinge would work. We have an external door, which we wanted to stop from hitting the rendered external brick frame. We put the stop on the bottom of the door. Works perfectly.

You could always add a small table or a plant to balance the space, this could also stop the door from going back too far and damage the mirror.

I would get the installation instructions for the fireplace you’re planning on having. They are usually available online and will have information on the surrounding construction required. Builders often have a reluctance to do something new, but they are also the one that has to build and warranty the work. You need to negotiate a position that you’re both happy with. If this column is just plasterboard and is just a cosmetic design feature it shouldn’t be that expensive. If the heater is installed as supported from the construction above rather self supporting from the base it may be a reasonable cost.