1. Disable the party leader switch button. Refer to the manual for the proper script call.
2. Just set the aggressive attribute manually. Once you do that, it will force it all the time and it can't be changed by the player. Again, refer to the manual for the proper script call.
3. & 4. Yes, you will need a custom script for that. You could do it with forced action commands, but it would be very bothersome.
5. You could do that with just a little bit of tinkering with the alignment system. You would just have to keep setting the party leader's alignment to the normal actor alignment while you keep setting the party members to an alignment that enemies don't regard as "enemy".