Was I out of order in not letting the car out? Are my (retrospective) justifications valid?
No. For your own safety I very rarely let a car out if I am in traffic, unless I am virtually stationary to begin with. If you are slowly decelerating towards the back of a queue, traffic behind you will be expecting you come to a halt at the back of the queue. The driver of the vehicle behind you will also be aiming for the back of the queue, and not expecting you to brake suddenly earlier.
Of course if there is no other traffic behind you then the car pulling out only has to wait an extra second for you to pass it.
