Are you sure they are not working? In order to test <C-d> and <C-f>, you need a documentation window that displays a lot of info (more than can be fit in the docs preview window). Only then can you see if these mappings scroll or not. For example:
With regards to <C-Space>, how are you testing it? If you go to insert mode, and hit <C-Space> does the completion menu pop up?
Edit: I noticed that you are using F and D for scrolling. I would rather use F(orward) and B(ack) or Up and Down to be consistent with vim’s default mappings. Here is how I have it mapped in my config file. Try this out and see if it works:
My bad careless didn’t notice that <C-d> and <C-f> are for the “documentation” section of the window. <C-d> and <C-f> are working well. Thank you. Btw, ‘f’ and ‘d’ are seem reasonable keys since they are close rather than ‘u’ and ‘d’, or ‘f’ and ‘b’.
<C-Space> is also working. <C-e>, closing the menu, makes to test easily.