Catacomb 3-D: The Descent no displays correctly the icons with faces for the supposed monsters locations on the maps of the Overhead Cheat (F10+O), instead it displays numbers and letters.

You're right, the (hexadecimal) numbers and letters in the maps of the Overview Cheat indicate the locations of monsters and items in the game. But there are not supposed to be graphical icons there. The original source code from the game shows that the numbers and letters represent how these monsters and items are stored in memory.

I think the Overview Cheat wasn't really intended as a navigation tool for players, but mainly as a tool for the developers to help in analyzing problems in the game. When you repeatedly press the Ctrl key in the Overview map, it will show various views of how the game handles the map in memory. Very useful for the developers, but not really for players. I think that's why they disabled the Overview Cheat in the Catacomb Abyss and its sequels.

Now I have a map editor that shows this icons on it. I can't see them in-game in Overhead as I wanted, but at least I know what is and where.

True, with a map editor like WDC, you can get a good view on every map.

Alternatively, if you don't mind using a source port to run Catacomb 3D, the latest version of CatacombGL can show a live automap in various formats.