Two Qs about campaign layers

[Hitman]

Hope someone can clarify this to me:

1.- Are all current layers needed (i.e. deleting one causes a crash) and can new ones be added (with new names) that will be picked by the game and loaded?

2.- Are any layers interrelated, i.e. are there layers that work by pairs and modifying one will cause weird effects on another? Or all of themcompletely independent and I can mix modded with stock layers?

Thanks