Premiere Elements has a couple of tools for controlling playback speed. Its Time Remapping tool is excellent and can be used to slow down an entire clip or only segments of the clip.
Time Remapping includes Frame Blending that makes for excellent, smooth slow motion.
Time Remapping can speed your clip up to 8x or down to 1/8x.
So could I play back 240fps video at 24fps, then use Time-Remapping to slow it down even further?
Or is the 8x slow motion the extent of what is possible with Premiere Elements, so 240fps could be slowed down to 30fps but no further?
The project settings for Premiere Elements only go as high as 60p fps -- even if you load a video with 240 fps into the project. But you can slow that down to 1/8x. It will still be 60p, but it will be one-eighth the playback speed.
If you need to slow it further, you can output your slowed movie as a video, add that output movie onto your timeline and then slow that to up 1/8x (which would be up 1/64x of your original video) and so on.