"She wants a square-top window with a flat-arch/segmental arch spandrel above it." Anderson and possibly also Pella make them. No one makes true arch top windows (except folks like me restoring houses). Poplar would be a horrible choice. While it holds paint well, it has a very poor rot resistance. The smaller movement with moisture is why softwoods are actually preferred for window work.

I would use a 60" wide x 40" high picture window with a 20" high RO operable transom above it. Then you could use the transom for ventilation and not have screens on the lower window. The wall above the transom would be about 7" tall, enough for trim and a ceiling molding. A structural header could be substituted for the second floor band joist/rim board. I've done it many times.

Google images for "floating mirror on bathroom window"; perhaps you'll see something that interests you. I imagine it would be pretty costly to replace that window with two smaller ones and a section of wall in between them in order to hang a mirror there.

Than you for all of these great points and suggestions! After taping off the space, I think a bay of casement windows is out of the running. Three windows would be too little view and two casements would have the seam right in the line of sight. You bring up a great point to be careful with muntin layout because one could potentially be right in the center as well. I am currently leaning toward a 3-panel horizontal slider with 14-28-14 size panels or one large awning window. My fear with the horizontal slider is that is doesn’t seal in the winter as much as the awning and could be drafty. My fear with the awning window is that since it doesn’t open completely, it won’t let in much air in the summer. Prices for each are about the same. We won’t have trim but rather a dry wall return. My husband says in obsessing (I mean, maybe a little ;)) but I really want this to be a feature piece!