I don't understand why you want all the framing. Either you want to disguise the geometry here, in which case I'd do all mirror on that wall, or as solid as possible, or you want framed mirrors in which case I'd have two separate ones and bring them in from the wall side to end up centered over the faucets. (Oval would help there.) Not clear what you're accomplishing by doing both at once. I wouldn't frame the window (just do a drywall return), and then choose one of the above approaches to the mirror issue.

If you want a marble backsplash, that should be possible, but there's going to be a very thin strip between the junction box and the mirror as sketched, which is probably what worries your contractor. Can you find a carerra tile instead of using a solid chunk? Should look just as nice and be much more flexible for handling this tight area.

Love where this reno is headed! It is smart to tape off like you are doing before fully executing. How about marble straight across and mirror over the outlets?

That photo is actually the one that I used to have my vanity built. unfortunately my space was not as wide and my cabinet maker decided to make it go wall to wall instead of freestanding like this one which created my problem ( and a few others). I thought about the oval or irregular shaped mirrors but wasn't sure if pulling them in would look right. Maybe that will be my next paper visual aid.....

If I don't do the framing on the wall, then the marble issue goes away because I don't have the space around the outlets that I need to fill - I can do a straight splash that will go under the outlets.

Thanks