at first install mediainfo to get real values of your videos
I only use mode 1 so here is my stuff about that:
if your source video for example has 1000 frames (pngs) dain can only interpolate between them
so the last frame can not be interpolated by dain
your interpolated_frames folder must contain 1999 frames
dain has the strange habit to render in step 3 with the factor between 1999 and 1000 frames
so it renders the outputvideo with factor 1,99 and that has the result that your outputvideo has 59,3 fps
here's a solution: try to copy the last frame of "original_frames" to "interpol_frames" after step 2 (rename it of course)
so that the factor of count of pngs is exactly 2.0 between both folders
if your source has 1000 frames and your interpol folder has 2000 frames all will be right
then run step 3 and you will get 59,94 fps
EDIT: sometimes it can be that dain forgets one frames additionally then you must add 2 frames at the interpol-folder
And don't enable the scene detection that will delete frames too