1) Make sure to enable AI Controlled for player combatants, e.g. in the combatant general settings for all combatants. This will have all combatants not directly set as the player (i.e. the combatant you move around) be AI controlled. Enabling AI Controlled Player will also have the player be AI controlled if you want that :)
You can find those settings for all combatants in Combatants > Combatants > General Settings in the Battle Settings > Default AI Settings. Each combatant can optionally override them.
2) You don't really need to switch it - a hunting move AI can still follow it's leader, it'll simply check for possible targets to hunt while following the leader.
If you want to let the player have control over that, you could e.g. use an ability, item or schematic (e.g. started via control maps) switch the move AIs of player group members.
Or, for being close to enemies, you could add a Trigger Machine to your enemy prefabs that changes the move AI of those entering the trigger (collider).