(Not an official response!)
I've looked at your screenshots, and looked at my FC's medium house in Mist, and I can't figure out what you're actually saying the problem is.
The railings look the same to me. They're going to be the same model copy-pasted around the room. It looks like a camera angle issue rather than one that's actually there. (With a possible side of character height/viewpoint - my short Midlander is going to be seeing it from a lower, more "2D" angle, and I think from your screenshots the wood strip might stick out with a sloping 'right-angled triangle' shape.)
It's also worth noticing that there are two alternating types of pillars along the walls: the bulky white ones and the smaller wooden ones like in your screenshot. They're different thicknesses and a symmetrical wallpaper image is probably always going to look off-centre as a result.
And if the horizontal wood strip is wider on one side than the other (as I *think* you're implying), what does that have to do with whether the towel rail will align with the vertical panelling?
Also I don't understand what your screenshot of the floor is supposed to show. Your rug isn't lined up with your tiles? That's not an issue related to the housing fixtures, that's entirely to do with your furniture placement and under your control, not the game programmers.
In any case you can't rely on the grid snap to get things perfect - it's designed to line your furniture up with your other furniture, not to perfectly fit every pattern of tiles or wallpaper. It's good for things like getting a vase in the centre of a table or lining up a row of partitions to make a wall without any gaps, but for subtle adjustments you'll need to turn it off and do it yourself.
Also I don't understand your last sentence at all. Floors? Windows? What are "they" and what are they hanging off?