Thanks for looking into this issue.
It doesn't really matter where the blank is: path or filename. It will not open when dropped.
As for issue #1, I specifically tested it with a file without a blank and I can't get it to open. As I did a fresh macOS install (so there is no leftover from a past version of SlickEdit) I've devised yet another test: I "Reset Window Layout" and guess what? With the "Default" Window layout it works!
So I closed all the Windows, closed SlickEdit and tried again: doesn't work. I've noticed that when I doesn't work, the dragged file does not get the "+" icon to indicate that a drop handle is present in the underlying window. I could not determine which Window combination causes it to work (I know it's a weird combination as I tried one by one and I know it's not a single Window).
So to reproduce #1: close all windows inside SlickEdit, restart SlickEdit and try dropping ANY file.
Edit #1: In time: once a file is opened, drop works even with all the windows closed (provided that SlickEdit is not closed). If SlickEdit starts with all Windows closed, it will take a "combination" (ex: file window or all default windows) of Windows open to make drop "start" working.
Edit #2: Did some extra testing and noticed another really WEIRD window combo that make it work: If the "Preview" window is DOCKED when it opens (as in the Default Window setup), drop works after SlickEdit starts. If "Preview" is FLOATING, it does not work. Sounds like there is some problem in the drop handle management.
The good about "Docked Preview" working is that I can have it "flushed" all the way to the bottom, and aside from the small "drag line", it behaves as it's supposed after a restart (I use a clean SlickEdit workspace).