TCmullet
In most cases, each frame (in my source) is different than the prior one
That's not enough. You need equidistant frames.
Good video is constant frame rate video with one time-delta between frames. Your video has not one time-delta. They are various at each frame.
If source have 25 fps then no way to get 15 fps without harm to time-delta. Such irregularity are present in your video it not at one place, it is everywhere.
no matter how far apart they are.
You are right. In your case you can get some result. But to get good result you need good source. Your source is bad.
I realize that if a framerate was really low, so low that any wild actions could occur between 2 snapped frames, then of course no lost action can be truly created.
Yes. You are right.
But if an arm moves from, say, pointing down 45 degrees from horizontal, then the next frame shows it pointing upward 45 degrees, I'd think InterFrame would be smart enough to generate an inter-frame with the arm pointing 0 degrees
No. Interframe can get 0 degrees arm at out only if sources has -5 and 5 degrees. 45 degrees are too much for this task. No rotation predicted. Only global shift and local shifts.
why does the ball, when appearing very discretely (round and clearly formed) sometimes get interpolated as 2 balls in one interpolated frame?
Because ball is small and his moves are large.
I have no idea how to help you. You need a lot of hard hand work frame-by-frame to reconstruct dropped original frames.
Such big intervals between frames for sports is very hard to any interpolator. Too small details, too large moves and differencies between frames.
Sorry. I have no time to do your job with your video. I must to work at new SVP version. Maybe after SVP release I will find a time to help you more.