As I recall, my original Blood (from the CD) game let you assign mouse buttons to specific functions...The "One whole unit" game does not. I cannot change the HUD either. All in all, the game doesn't really work or respond very well compared to the original.
Perhaps I am missing something here???

I assume you already had a looksie over in the dedicated subforum (www.gog.com/forum/blood_series) for anything regarding the described problems?

Nightdive Studios' Remaster will probably take care of those and other issues too, if there's no solution in your case.

m210.duke4.net/index.php

Try BloodGDX. Works wonders for me.

far out, I didn't know about that.

HUD visibility can be changed with the +- keys (or the 2 keys on the right next to 0).

And IIRC, you can remap the keys by running the setup that is located in the game folder (it's an old DOS setup program). Not sure about the mouse though (never played with a mouse).

You should also be aware of the difficulty bug. Here is a link to a thread with a guide on how to fix it.