Current questions and answers regarding the map editor.

If it’s necessary to film on two levels (both on the surface and in the Dungeon), then part of the surface will have to be designed as a Dungeon. Fence off part of the map with underground walls. You can see an example in the dialogue
DialogScenes\A2Single\SM2\S2\DialogScene.xdb

Added 2 minutes later
The second question then becomes irrelevant.

Added 7 minutes later
If the surface is not needed, then
1. Open the map in Notepad.
2. Change the InitialFloor parameter to 1.
3. Close.
4. Save.

When you open the map/dialogue, the Dungeon should be displayed.

Then set the height to ≈17.