Can you remove the chair rail and do your favorite throughout? I had a chair rail in my powder room — don’t ask me why the previous owner installed it there— but I ripped it out to wallpaper and hang a mural.

Maybe add panels, or faux panels. Lots of different looks and methods.

With art and fireplace mantel above the rail, what about wallpaper/ pattern below the rail?

Do you mean alligator texture below rail?
Or different pattern below
As you can see in my pic I tried a Ralph Lauren pattern below

They are too different, do you think?

Yes I meant patterned below.

Why wallpaper? If it was me, I would consider just painting - maybe a sueded paint. With paint you can change your accessories as the years go by without the expense/trouble of removing wallpaper. I like the texture of the wall paper and maybe the wallpaper on the bottom and a coordinating green color would be perfect.

Thank you ......that’s something to think about

Seeing your curtains are plaid, what's the plan with plaid and other wallpaper?

This room is so beautiful wow!! Truly magnificent!
Number one I would consider wood paneling matching the molding color.
My concern with the alligator wallpaper is the style seems more modern to me. Not that you can’t have it in a traditional space. Just this room is very traditional. Would you consider using it in a different space? Maybe a bathroom?
What’s currently on the bottom half of the wall now? Very pretty... maybe just do some woodwork on half the wall and do a similar wallpaper on the other half.

I think your current striped wallpaper works below the chair rail since it kinda simulates wainscotting. I think you could 1) have alligator wallpaper on top and paint the chair rail and below green, 2) alligator on top and add some paneling or grasscloth wallpaper on the bottom half, 3) do away with the chair rail and do the whole room in the alligator wallpaper. Like nmmamax3 said, there have also been some cool things done with specialty paint lately. Portola makes a roman clay and a limewash paint. We're using the limewash on our new kitchen hood instead of plaster. You have lots of options for this cool room!

Love the wallpaper that you have proposed! I would keep the chair rail and the wood is lovely. And, then, choose the darkest tone from the wallpaper and paint that color below the chair rail. See how you like it. If you feel it needs “more,” keep looking. Think of it as a work in progress. Are you keeping the window treatments in the room? They are rather bossy. That is why I suggest paint, rather than another pattern, beneath the chair rail.

I love your wallpaper choice too. And I would opt for painted wall rather than another paper. Are you considering further updates? I think new curtains and rug could really update things. Possibly a hide rug or evdd egg n a sisal with hide layered on top.

Really like the Thibaut wallpaper you've chosen. Are the drapes, rug, furniture staying the same?