Levelling and original duration changes

Do not think of how the software works. Think of planning and scheduling perse and how to do it properly. When you delay the start of a certain activity due to availablity of resources, of course you will delay its planned finish date as well and so as its successor activities (if any), but not necessarily increasing the individual activity durations but perhaps increasing the total project duration (considering there is no Float). 

On the other hand, if you have actually started an activity and the execution of same activity is delayed due to availability of resources, then consequently, you will increase its duration (again, forget about how the software works), and of course the start of its successor activities (if any) shall be delayed, but not necessarily increasing the durations of the successor activities, only the planned start and planned finish dates shall be delayed. Again, perhaps increasing the total project duration if there is no Float. 

Where am I? What is your question again? Oh resource leveling... On the same token, if you have assigned resources to your activities, resources will just follow exactly the behaviour of the activities (if you have assigned the resources properly, considering shifts, calendars, units etc.). For activity-logic driven. For Resource Driven or Depenendent - would be a different story. However, I guess you always plan activities first and then assign your resources. 

Just use the software exactly on how you want it to behave. It can capture exactly all the information you feed in but will never tell you when you were doing it right or wrong. "Use the sofware and not the software use You". 

you can try using the constrain tas late as possible 

Thanks for the laugh about management in your company!