World Editor's menu bar->Scenario->Map Options->untick Masked Areas are Partially Visible.
Then, in the Trigger Editor, Actions: Visibility->Enable Black Mask.
Sure but first, instantly apply a camera then make it move because otherwise you'll be stuck with it panning from one end of the map to where your hero is.
Make sure you turn the trigger off once it's supposed to run.
What do you mean it bugs? Just turn cinematic mode on and don't disable user control until after you press a dialog button.
More optional quests but you should have made the map a little bigger than it is. There's still some space however.
I don't quite get it. You should mark it as completed after the demon dies. It's nothing special.
Trigger Editor, Actions: Selection-Clear Selection for Player after Disable User Control. After Enabling User Control->Selection-Select Unit for Player.
Your game cache is probably not saved and ported properly for the next map.
Making a Basic Campaign
How to use the campaign editor