get the plastic lid of an icecream container-cut the thick edge of 3 sides. slip the lid right under the edge of the mirror, and it will stop paint getting on wall. we use this for skirtings all the time, works a treat. just remember to wipe the back of the plastic every now and then, in case you collect any paint on the back.

I feel it's too heavy looking for your room. You say it's a very large room, so you don't need it to make the room feel larger. Perhaps look for a picture/painting.

Can anyone tell me best height for tiles on vanity to use as a back splash I’m using 3 mirrors above I was going to drop mirrors to vanity but don’t like the look

Depending on latches and fixing area, etc - you could get a good glazier with a bit of finesse to custom make a mirror to fit into the window area, and then just centre the vanity below the window. Provided the window glass is obscure, it will still look like a window from the outside, but you'll have an interesting recessed mirror in your bathroom! (hope that makes sense) But, If you want a seperate shower then it looks like you'll have to alter a lot of plumbing anyway, so you might as well move the vanity while you're at it.