Correct me if I'm wrong but Constantine has always struck me as quite mercurial, much like his brother, only in a much more slapdash, indecisive way. It could always be the case that he dawdles and plays Hamlet on the Neva trying to decide on a course of action until Napoleon has imposed a harsh peace on Prussia?
Not that I’m a big expert on Constantine’s personality but while his father was, indeed, compared to Hamlet it was mostly related to the circumstances of his father’s death and his mother’s behavior.
As for being mercurial, I would not agree that this applied to Alexander: obviously, he changed his views more than once but this was mostly a pure opportunism domestically (it seems that only later in his reign he went into all that mysticism) and especially internationally: no matter what he could be forced to do and declare within a current set of the circumstances, he was staunchly anti-Napoleonic even when forced to claim him a friend.
Constantine was mercurial on the minor issues, mostly behavioral ones, but on the bigger issues he was pretty consistent: his pro-Napoleonic position lasted for years and changed only at the face of a direct confrontation. His obsession with a drill also lasted through his life. He could be elusive
when this suited him as was the case after the death of AI but his exchange with Nicholas at that time looks quite intelligent: he was clearly minimizing a personal risk without burning any bridges.
I’d
guess that his behavior in your scenario would involve less rather than more thinking because
from his perspective situation is beautifully simplistic and ideally clear:
1. Napoleon is ally so if he is attacked Russia is under obligation to come to his defense.
2. Prussia was under Russian protection and supposed to behave as the Russian client but it decided to break this dependency, clearly to the determent of the Russian interests. It also broke a peace treaty with Napoleon. It must be severely punished for its double betrayal but in a way that does not rock the European boat too much, aka, there must be a “kingdom” with Hohenzollern king.
3. Russian interests must not suffer, which means that the Duchy should not be excessively expanded,
especially on the Baltic coast.
4. The boring details can be discussed later with his dear friend Napoleon in the terms of a mutual understanding. If Prussia (and Pomerania) can be made into a separate state ruled by a friendly/relative German prince and becoming a de facto Russian ally - perfect.
😉