The Right of Assassination requires the limitation of alternate claims, no matter how weak, to the Iridium Throne. Having a battle fleet in orbit is a clear designation that a change of authority has occurred and typically would impress stability upon the Moot.
Dulinor lacked the staying power to take Capital. Perhaps that is not where his heart was drawn. Perhaps moving the throne to Dlan was more in-line with his approach.
Impossible to bring a battle squadron along without bringing up an alert, if not telegraphing his intentions (and signing his own death warrant). The plan was to claim the throne by default by eliminating the direct heirs. His relatively recent appointment by Strephon, who also made the Domains more than just nominal and/or ceremonial anachronisms also points to growing influence, but not a man bred for such a position of high influence.
The plan was to stick around. The plan didn't survive contact with the enemy. Dulinor made a bad call. Such things happen.
One might suspect that Dulinor was the kind of person who assumed that all things would work out as planned. The whole plan was predicated on the assumption that the rest of the high nobles would accept the Right of Assassination as valid even though it had been 500 years since it had been invoked and people apparently had forgotten all about it or regarded it as a quaint bit of historical trivia.
Just like the Domains.

Which had been relatively recently revived by Strephon as a result of Longbow revelations of the incoming Empress Wave and a plan for the Imperium to survive it (which ironically enough is what started the chain of events to bring it down - or properly prepared the Regency to weather it, depending how you look at it).
From the whole Kulligan and Jurisor bits in TNS in Survival Margin, he seems to delegate an awful lot of authority to spook type Right Hand Man. Clearly that's a certain feature of any decent command structure, but the way Kulligan rebels and Jurisor is defiant, if not truculent while still seemingly loyal in the face of Dulinor's recriminations over the assassinations doesn't speak to a firm command personality. Certainly not the best judge of men putting such in high positions that he experiences such disloyalty and/or dissonance with them.
It redoubles my impression of Dulinor being a Cicero-style "New Man" Which would give him a chip on his shoulder with the rest of the aristocratic 3I nobility (who supposedly were scornful and/or opposed his rise to at least some degree), added to the chip he would have by being a non-Virasin on Dlan. If his background was military, which makes me think of him more as Pompey Magnus than a Caesar (who was a born and bred aristocrat, of course) except without the famous pattern of victories, but just as resentful.
Meshing in the retcon Gurps material on Isis, one could imagine her having at least a similar level of influence, if not more. That makes Dulinor a man pulled in multiple directions by those closest to him who humor his vanity in seeing himself as the man to reform the Imperium and to feather their own nests at the same time. His populist rhetoric and the reformation of the Domain into the Federation of Ilelish as a proto-Regency speak to his goals. Was Isis a voice of moderation or was she urging him on to more or was she just of like mind has to be left to YTU as insufficient data in OTU.
Dulinor seems to have been rather obscure prior to his elevation by Strephon, so doubtful she was groomed her life, as was Iphigenia, for example, so she can be a bit of a wild card.