To play a different attack animation, you'd have to set up different attack abilities to play them - unless you e.g. use a Wait For Input node in your battle events to check for the 2nd input and play a different attack.
Key combos are just input keys that receive input after the defined key combination was used, the rest comes from your control map setup and is still restricted to only allow using an action when the combatant is able to.
If your combatants are moved via the move AI, you can e.g. block it during the entire battle event by enabling Block Actor Move AI in the Event Settings node. Or use a Block Move AI node (enable Use Object to block it for a defined object, e.g. the User actor).
For the player, there are control block options in the settings of the battle system type you're using.
Your monsters should have colliders, otherwise they wouldn't be able to collide with stuff (e.g. the ground :D). Does the ground have a collider?
