Prescient. "A mere demarcation on parchment" is not enough. The constitution, in and of itself, does not protect us. We, the voters, must be educated and vigilant. Thank you for this work which is forcing me to be clear about my own thoughts.
I think he did see that the competition between the branches would be so stiff. His solution was negative checks on power. That is, unless all three branches agreed, a bill could not pass a veto or a law could not be declared constitutional. All three had to agree. They could negate the other two but not do anything on their own. Negative checks.
There are two obvious failures. First, with the Judiciary "legislating from the bench", for example extending marriage to gay couples, something neither the Legislature nor the Executive agreed to. Second, the Executive creating laws through the regulatory process with the acquiescence of the court, for example declaring CO2 a pollutant, without congressional approval.
What I think Madison did NOT see was that congress would willingly give up its power to the executive by creating the regulatory process. Both Republicans and Democrats have decided to do this because, I believe, they want to pass laws without really knowing what they are passing. That is they can go home and claim "I voted for the Clean Air Act!’ and not be held responsible for the result of their vote. The Democrats have decided to do this because they know that they cannot know the unintended consequences of their excessive Socialist laws and need an singular executive to change the laws as consequences are about to occur.
I think that what threw Madison's calculations astray was the advent of party politics - such things as political parties within the United States were yet to form and Washington, for one, hated the very idea. If one's primary political identification is with the branch of government in which one works, there is a natural tendency toward defending one's turf. Throw party identification in there and it's another story. A Senator would, ceteris paribus, have no reason to wish to stretch a little political power just because a President wanted it, but a Whig Senator might want to do so for a Whig President, or a Democrat for a Democrat. Someday soon even the Republicans may master party politics.
You can see how that upsets the balance. In come ways it may be a good thing, but what it does to the separation of powers isn't one of them. IMHO, of course.
We'll see in the next paper how Madison does foresee a dynamic within the Virginia constitution that party politics did end up subsuming, in the form of how few people it might take to prevent the remediation of an untoward assumption of power in that government. (Cue Announcer Voice: "Tune in Thursday for another thrilling chapter...") :-)