I'm not familiar with BM paints, but SW has a soft yellow called Jersey Cream. Another popular SW shade that is a little more neutral than Jersey Cream is Ivoire. I've read that if you want a "soft" yellow, look for shades more in the cream family. Yellow has a way of intensifying once it's up on the walls.

It would be helpful to us if you could post the picture of the room you saw on houzz so that we could see what you are seeing. But, without seeing the room, I agree that a white would be a good choice that will serve you well in the future if you decide to change the wallpaper. I would not paint the walls a gray because gray is fading in popularity. Any time you see a popular color such as gray EVERYWHERE, it's a warning that the color will be out of favor in a year or 2 or 3. I know that when I get together with other designers we all voice our tiredness with gray and are reluctant to specify expensive merchandise in gray. If you want a little gray in the room, use a couple of pillows which will not be very expensive to replace later on.

Is your pic a picture of your space in question? Are you saying the toilet will be housed outside of this space? I can't imagine a shower and sink opened to a bed area and a toilet housed somewhere else. If you're putting work into tile then I feel it's as important if not more that you rethink the whole idea of a guest room. Is there any way for you to wall off a section in this space to house a toilet, sink and shower and have it NOT be a part of the bedroom space? If I were your guest I wouldn't care about tile. I would care that I'd have to go outside to use the toilet AND shower in the openness of a bedroom. I as a guest however wouldn't mind going through the shed door to another small 'closet' of sorts that houses a shower, sink and toilet.

since she's under 2, I don't think you have to worry about a desk yet. I'd have fun and redo the walls and bring in accessories. Doing the wall by her bed, and/or the opposing wall would be nice. here are some ideas: paint the walls a soft blush/mauve (or whatever color you like), get some flower decals, shelving, little chairs board and batten? some peel and stick wallpaper? a cute swinging chair? some fairy lights? I think this one is pretty. one wall w/the paper, and the other walls coordinating color. bring in some fun play objects like the tent w/the furry rug, little lights, stuffed animals. for that bare wall, I think you could set up a doll house, or toy kitchen, tea set, whatever she likes. maybe a reading area? display the books.