The 6000x1000 px size will affect only the panorama photos, other photos are constrained by the 1000px height.
Yes I agree.
You can manually mark panorama images in the Edit mode / Image data panel.
That is what I did. (as highlighted in yellow on the attachment)
The panorama images start with the centre, as that is the most important part of an image, and anyway, all the images are centred in the lightbox.
It is a moot point about which part of the image is most important. However. in a long group photo, it starts in the middle and pans to the right and stops. This completely misses out the left-hand side unless you introduce some movement by nudging with the mouse pointer. It then oscillates from side to side at tremendous speed.
The pan speed is constant, there's no special setting for that, but I can introduce some control for that.
That would be very useful. (to slow it down)