How do I activate Dopefish mode? I can't type in the command on DOSbox because it refuses to change to the game folder, when I use its shortened DOS name and I do not know how to mount a folder within another folder as an attempt to workaround the issue.

The shortened name for my Dark War folder is Rise~1.

Not sure if this would work on latest versions (and for all platforms, I'm running it on OS X), but try looking for a file called bxlaunch.bat in your ROTT folder.
Open it with a text editor, you should see this at the end of the file:

ROTT.EXE

Simply change that to:

ROTT.EXE -dopefish

Next time you run the game it will activate Dopefish mode ;)