turned off magnet, closed the program and restarted it

Why? You don't have to restart when switching magnetism on or off. There's a nice button with a magnet in the toolbar.

but I see no way to rotate my collection of windows horizontally before the 1st export

If you look at the image from GaudiGalopin with the 5 segments you can see that the windows are horizontal. Just resize them to fit in the horizontal composition. I.e. small height, wide width for the rectangular window. The "Fixed window" is a nice window to do that. The windows are only there to create the hole so what they look like doesn't matter.
Set the width and height of the round windows to the same value as the height of the rectangular window.
After placing the windows you export the walls. And after import of the walls you can tilt the walls to the correct position. You don't tilt/rotate the windows, only the walls after expor/import.