job must finish by

I think it is the network architecture problem rather P3 problem. To indue a project must finish on, at least 2 level can be control and either one of them can perform: at project overview and/or to the last activity.

However, if both client and contractor are in different sub-projects and merge to a master project, there will be another story.

More over Your Client accepted to have in the scedule activities with constrains (like must finish, must start, early start or finish, etc ...).

In my experience they accept only in the interface milestone to compare with another milestone at the end of the "work package".

If for example You put a must start on 10 april for a pole installation, it mean is missing all the logic for the procurement and delivery of it.