[JavaBrewer]

Many homes built in the last 20 years face similar situations. Home designers incorporated media niches and similar cut outs for the older big box TVs. My last home was the same - had a dedicated niche for a big box TV.

Most of your divisions are not load bearing or structural and could be easily removed and redone. Not so sure about the two walls rising to ceiling above your FP - which I assume from the picture is a pre-fab plumbed with gas line.

Dead simple to simply frame in the mirrored opening to be flush with wall. You can lose or simply resize the other decorative spaces to manage for utility and aesthetics. Some simple studs, screw gun or nailer, some sheetrock, mud and tape. Should not be very expensive depending on your desired finished goal. This still does not address the TV - I'm not a fan of mounting above the FP - too high and candidate for heat damage. Will also be an eye sore. Perhaps rethink where the TV would go.