arexma, you could not be more wrong. but fortunately, i have already found a solution
(I had written an incredibly detailed response to this, effectively explaining the whole process, but i screwed up and pressed the ‘reply to thread’ button and lost it all. will i try to repeat it? maybe.)
So i start out with, say, 120 frames, as in my actual test. it just so happens to be a fifth the speed i want it to be, so i must make it faster. i have a choice of an incredibly simple click to create a ‘speed control’ layer, or i could make an incredibly complicated setup to add some realistic motion blur in as a side benefit. which one will i choose?
you know which one.
i start by making five layers of the same video in the video editor, each one at 20% opacity (explained later). I then offset them by a single frame for layer 2, two frames for layer 3, 3 for layer 4, and so on until i have the factor i want to speed the footage up by (5).
now, if i go to frame 5, i should see a combination of frames 1, 2, 3, 4, and 5 at the same frame. this is because five 20% opacity layers combined make a 100% opacity composite layer. i could then go to frame ten and see a combo of frames 6, 7, 8, 9, and 10. you should, by now, get the idea.
setting the step (skip frames) value to five (or, 4, probably), making the end frame at 120, and rendering at 24 fps will give me a one second video, sped up 5 times from the source, but also has 5 samples of motion blur.
so, in short, its not frame interpolation. i dont need to interpolate, because i have an ACTUAL slow-motion video. that was intentional from the start.
it is actually the opposite, frame COMBINation. i think that sums it up well, dont you think?