#1: Correct, because the critical path typically has zero float.
#2: CAN (but doesn't have to). It could be ANOTHER critical path (you can have more than one).
#3: CAN (but doesn't have to).
#4: Correct. You are just increasing the float.
#1-2: You can also use fasttracking and crashing.
Crashing - increases costs (more people / overtime).
Fastracking - increases risks (we are painting and laying carpet at the same time)
Resource Smoothing - (adjusting resources so nobody is working over say 85% productivity).
Resource Leveling - adjust start / finish dates to adjust resource supply with demand (** can cause critical path to change)