bath
Fixed shower panel
https://www.fabglassandmirror.com/cela-910.html
Thoughts on Gavin's bath. Glass tile in a blue/grey glass tile that compliments his counters? Small or large rectangle. Sam's bath will be easier, just may need to repaint.
Darzy
I agree with Natalie that a white tile will make the floor look dingy. How about 6 x 12 shower tile in a blue color that complements the floor tile? Then, you can do the shower floor a darker blue. Since the colors will contrast, you won't have to worry about a dingy look. I'd nix the marble counter for your vanity. I prefer "like kind" materials as the beautiful marble may accentuate the "dated" floor tile. Photo of blue shower tile. (Since you like blue)...
tile
Q