Been there...done that. I've moved a central light to two side lights (and oddly due to my wife's redecorating ideas back again). First word: if you have a splice, it needs to remain accessible. So if that box still has a live connection in it, you need to leave a cover plate (though you can paint or otherwise hide it with anything as long as you do not need to disturb the "finished surface" to get to it). The first thing to do is see if you can find where the wires run now. In my case I had the good fortune of having an accessible attic above and was able to pull the wire from the central box up and then fish it down to the closer of the two wall sconce locations. I then ran a wire from there to the other location. It wasn't necessary to cut any holes larger than the box size for the new sconces. Since the old box no longer had any wiring in it, I could cover it over with wall patch. If you're not so lucky, you'll probably need to be cutting into the wall to find and relocate the wiring. If we're talking drywall, this isn't a big thing. I've spent more time cutting and patching drywall over my life than I care to think about. You may wish or need to remove the mirror. Hopefully it's mounted reasonably rather than being glued to the wall (in the latter case you'll have a lot of drywall to replace :)).

no, it wouldn't I mixed finishes in all my baths..looks pretty natural. I don't even think people notice..they just see bigger picture. Don't mix brushed nickel and polished nickel, or chrome and nickel, these are too close to mix successfuly. ORB is different enough from brushed nickel, and also gives that very neutral feel, it being dark..you should be fine. but you can also cross post your question in Home Decorating forum..there's usually where questions about colors etc. go. Or Bathrooms one..but Home Decorating has more traffic..you 'll get more responces

Get better ventilation before getting the next one. The pitting comes from moisture sitting on the fixture , I have never had that problem because bathroom ventilation is always a top priority.

I like equally the two light fixtures Sabrina shared. The first one does look more delicate, which kinda goes with the marble, but the second is more unique. I'd like to see the mirror framed ... it looks kinda plain next to the marble.