i agree. 100%
When I tried to talk to people about this same idea last year, I got a lot of resistance of all kinds, from everywhere. But in the end all that resistance 'melts' away when you piece apart the arguments.
The hand-shower would have a concealed flexible line. Better terminology can be used to describe the 'supply line' but off-hand I don't remember it right now. Later, if we keep talking, I'll get it. You really need the right terminology if you want anyone who knows plumbing or bathrooms to agree with you. My opinion.
A bathroom is not a kitchen. In a bathroom, water can spray from anywhere and land anywhere. Is your tub a tub-shower? If so, water will definitely get into the "collar" where your flex supply line to the handshower will come out of the deck or wall.
This is the biggest impediment to getting people to agree that you "can" do what you are suggesting. My opinion.
Second question, much less serious: -your "elbow" must be placed somewhere ideally where you can see it and access it. Which elbow? The one that starts the flex supply line. Usually it is on the finished side of your living area, on a surface, and NOT "hidden" behind the wall or under a deck. Sczary thought to some, but easy to conceptualize for me and you.
Hope this helps.