I wouldn't try to align it to the frames. I would instead consider centering the fixture's height on the height of the wall area above the transom windows. Quite similar to the existing light fixture I think.

No, it is custom. I went to the store and talked with an associate. They make it to the exact length. Also, because it is being hung on a slanted wall, I will need an adapter.

I read that a chandelier can be at a height where in you raise the chandelier one foot for every three feet of height to the ceiling.....plus, if it is custom would the fabricator have an idea for exact height. Plus calling Curry & Co might give you answers for exact placement and You Tube is a great place to get ideas and opinions....I would start with Curry & Co.

Are you also replacing the matching sconce on the stairway? If the chandelier is custom, perhaps there are sconces to match?

Yes, definitely replacing that.

Call curry and co.