Any comment on using a mirror like this

we need photos

Without pictures all I can say is that in general it's better to put art over a fireplace. But we don't know how high your ceilings are, what your mantel looks like, whether mirrors would reflect anything attractive, etc.

As a rule of thumb I advise only use mirror when it reflects something of interest. If all you see in blank wall being reflected you might as well not put anything over the mantle.

Get a roll of cheap craft paper and cut out mock-ups of the mirror and then tape them up to the wall.

I agree with others, that art will probably be better than mirrors.