3. being able to specify that a particular image slot scales down to 50% on export would be much better. I don't want to scale every image slot, just the background ones.
This suggestion is actually a good one and would also be one of the feature I would suggest if no one else is mentioning. I also do this all the time using different scale for different parts to compose scenes. e.g. background, shadow, sun shine/lights are usually large and could tolerate blurry. After a scene is done, I usually resize those images in the image folder and change the scale to 2x in Spine. (note that Mesh may have issue.)
For an animation, some parts may not needed to be detail but they could be large.It make sense to use a lower resolution image to reduce export size. If spine provide an "export scale" option ( like 25%,50%,100%) on attachment panel to control the local export size, it would definitely save me more time and effort.
I would also suggest removing the BG images from your spine export and add them in the engine instead. That way your atlas is only for animation and the engine can do the compression of the big BG images it's own way.
Only work If the background is one big still image. A background could be composed of multiple animated large parts.
For an instance, I worked on ship scene where the background is animated sea water with a still wood platform and a large floating ship on top. Most parts are dynamic but still need to be scaled down.