If you use a premultiplied video file, like prores 4444, is partial transparency possible?
The file I got is transparent in a sense that if I put other graphics behind it, i can see though. It is also noticeable in the preview that there is transparency.
Playing the file on the key fill channel, no transparency is visible.
Is this normal behavior and is there a fix (for the editor) to still do this with a single premultiplied file?
Broadcast video does not support transparency, that is the reason why you have a separate output for fill (colors) and key (alpha channel). If you watch the key output you will see the transparent regions as black and the opaque regions as white. Any shade of gray 
denotes a half transparency.
I expected as much. Playing this pre-multiplied file on a fill key channel seems to not show any partial transparency. It does actually key, but not partially. Yet in the inspector and in the preview it clearly does show.
I am not an editor, but I expect that the file delivered is not as it should and it has to do with not having the transparency on the key channel.
You can easy check that by looking at the key out. If it is full white it does not have any transparency.