All your arguments are essentially “just because he does not show it, it does not mean it is not there”. I am sorry, but it does not work like that when you are analysing things beyond your personal head canon. The fact is that there is exactly one instance in the whole OT where Vader visibly looses his composure (i.e., “Tear the ship apart” scene), and tons of instances where he keeps his composure despite strong challenges and provocations. And no, cold-blooded execution does not imply anger or loss of composure, especially when one is doing it in such a calm manner accompanied by witty comments.

If you make a comparison to PT Anakin, who whines and has rage outbursts at every slight challenge, OT Vader is a complete opposite. These two characters just cannot be the same person, unless dark side improves ones character in an extreme way.