1a. Only select the units you want to attack with when you attack.
b. No, just don't select the whole group.
c. The group formations are useful for arranging your line.

2. I think the infantry is designed to wheel before firing.

3. React ... how? In general, units will try to do what you tell them.

4. I think the unit position is a pretty good indication of area of fire.

5. You can waypoint movement with shift but not actions, as far as I know.

Groups are mainly useful for preserving your custom formations when moving, or for organising yourself.