Chaos Bolt do you pick a die AFTER you roll the d8s or BEFORE?

Shadows of Moil heavily obscures with everyday darkness and therefore I can see with Darkision?

Would it be acceptable to select an appropriate ability for the spellcasting ability depending on the circumstances?

Open Hand Technique feature to stop Shield

If an Open Hand Monk hits a creature with the shield spell prepared and uses it's Open Hand Technique feature to stop it from using reactions until the end of the Monk's next turn, would the creature be able to cast shield to cancel the attack?