disclaimer: I'm not that read up on the Rebellion fluff, so I might be missing stuff here, but my 0.02Cr on the matter:
I aggree with the general feeling that the rebellion, as written, feels forced, and the cracks that opened so wide just didn't seem to exist in the preceding material that we had to work with (saying the "imperium is not a strong as it appears at first glance" is not the same as saying the "Imperium system is so fragile that a single bad succession could rip the whole edifice apart".) to this, all I can suggest is that most background material is focused on the peoples of the Spinward Marches, a border zone, and one that has a clear external threat to unify against. They may be much more unified and have a greater belief in the value of the Imperium than the citizens of the Core that have no conceivable external threats, It's already noted in fluff that the domain of Deneb has something of a different culture (look at the use of "behind the claw" as a label, which implies a stereotype to associate with the label).
on Lucan and his inability to beat Dulinor, I aggree with others that their might well have been a element of "the imperium is Too Big To Fail" in the thinking of many, much as it played a part in the fall of the 1st imperium.
another factor that might be the speed at which the initial events happened on Capital, which might have sown many of the seeds of distrust.
Due to the time lags involved in interstellar governance, the Moot and most nobles were quite likely used to having the luxury of time in order to decide what to do, because taking a week to deliberate didn't add that much lag to a order that took six months to reach the frontier, and it would take just as long for the results to filter back. Hell, they were likely used to rubber-stamping the actions of the nobles on the spot because the local duke had already made his decision and was months into implementing it before they even heard about it.
Then, suddenly, if the space of a few hours, several of the foundational assumptions of the game are overturned. The Emperor. the ultimate arbiter in their power struggles, was dead, as was his established Heir, and killed by a man who they mostly thought of as one of the Emperors allies.
This assassin, their peer and a fellow player of the game, then demanded that they recognise him as Emperor, claiming the throne and "winning" the game, on the basis of a rule not used in 500 years. Naturally enough, the moot thought that this, for lack of a better word, was cheating, and refused to condone the power play that broke almost every rule they knew about the game. So Dulinor flees, having misread the Moot totally.
Then, Lucan turns up and basically demands the same thing that Dulinor did. At this point, He's already declared himself Emperor, started rattling off orders, and is really just waiting for the Moot to rubber stamp his accession to the throne.
No one is really sure what us actually happening, who is involved and what their end goals are. Lucan might be in the line of sucession, but what happened to his brother? Could all this be a power play by Lucan? The Moot is using to having time, time to think, to plot, to scheme, to bargain and compromise, but now it hasn't got time for any of that. The Moot is being presented with a fait accompli. Lucan is now emperor, loyal citizen. please sign here on the dotted line, and I can get on with the business of crushing this revolt.
But the Moot knows, not just thinks, but knows, that they choose the next Emperor. Lucan was third in line, true but thats because someone had to be, and no one seriously expected him to become Emperor*. Their must have been half a dozen nobles in the Moot who, arguably, were better suited for the Throne than Lucan.
Thus, they hesitate, and try and delay, because they desire time, time to understand what is going on and for the situation to crystallise.
Lucan, on the other hand, sees things differently. His world has also just been turned upside down. He has just lost several close family members. A man he trusted has just committed the ultimate betrayal, and he has just survived a attempt on his own life.
The time to act is now. He knows that Dulinor is already on the way back to his Domain to rally his fleets and start a war. He needs to get the Imperium moving, to mobilise it, before any more time is lost and Dulinor can march on capital.
But the Moot is hesitating, quibbling, and their are rumours that he had something to do with his brothers death**. Why are they stalling? I am the rightful heir, unexcepted though my accession is. The imperium is facing a crisis, and needs a strong government to direct it though the storm, so why they blocking the only man who can give them that? Are they in on the plot? are they also traitors? can I trust any of them?
Lucan might not have been paranoid before, but he's just had a deeply traumatic event happen to him, and he wouldn't be the first to have trust issues afterwards. And, you could argue he wasn't really that paranoid afterwards, considering the number of people who really were out to get him.
This inability to trust others is likely the foundation of the micro managing aspects of his personality. He likely knows that its a sub optimal way of doing things, but he just doesn't trust his underlings anymore.
*although I must question the fact that the Emperor, his anointed direct heir, plus his de jure second and third heirs, all lived in same building for 10+ years, and no one thought that this was a bad practice. plently of things the 3I could have flattened the Imperial Palace and everyone in it, and not all of them require malicious intent.
**is it ever established if he did or didn't?