1) How to rotate a camera in the game mode fully around a player?
2) How to create a dungeon or cave? Are there anything tools for these task?
3) How to add a skybox to the scene?
1. Not sure how to enable it in Divinity Editor's in-game camera. As mentioned, a (not to convenient) workaround is to test the mod in D:OS with the relevant game option selected.
2. I don't think you can make terrain that it is directly above other terrain on the Z-axis; so, like, a cave floor and the cave roof above it, cut out of/into a mountain on your main game map wouldn't be possible. Options are to instance the cave interior to another part of the level, or you could experiment with making a cave roof using in-game items with clever positioning and rotation (I'd be amazed if this didn't look terrible!).
3. Check your Atmosphere/Light settings (the sun icon) and go to the Varia tab. There's a SkyDome section in there.
