Dulinor is not so much impulsive, and inept at timing his coup.
Dulinor had populist reforms in mind. He was willing to do anything for the good of the imperium, and he saw Strephon was no longer a strong and effective leader, and could not see the strength of the reforms; if Strephon wouldn't, it fell to him to depose Strephon and impose the reforms himself.
Dulinor is a zealot and an idealist, and even somewhat practical. He's not going to order the assassinations, nor will he inquire when he finds out they occurred.
He should be seen as impressive, and decisive, even if not always right nor fully prepared. He is rightly to be feared, if you oppose him.
And the domain of Vland is too important and too much a core to be left to itself, especially with all the extra-domain assets they'd have lost during the War, and their pension for long memories.
And they could be much more easily crippled than captured; bring in a huge fleet, pinpont a manufaacturing world, destroy ONLY the factory districts (the maps are on file, after all), and repeat a few times. And each time, offer rebuilding aid if the planet as a whole votes to switch sides. And keep hitting those systems every few months to prevent rebuilding, and to prevent ship building.
And remember: Meson guns, even factor A-F are 100-150m spheres of annihilation. Dulinor WILL use them, plus economics, to bring the Bilandi under control; likewise, he's smart enough to let their commerce flow as soon as he's got control, and gets himself elected shadow emperor of the RZS... And lets not forget: if Nasiiraka goes through 5 CEOs in 10 months, all suspiciously, #6 is likely to be somewhat more anti imperial, but he's also likely to realize that if he doesn't ACT that way, he's less likely to be accident prone.
Lucan was mad, and random, and petty.
Dulinor was Cold, ruthless, charming, and fanatic.
Both should scare you. But at least, with Dulinor, you don't need to worry what color underwear you can't buy this year.