As there doesn't presently seem to be much interest in killing Lucan, I'm going to digress a bit.
Yup and they did with the very thing that made him a candidate is premised to weaken his strength from the very beginning of the MT source material: suspect to the murder of his brother which led to rumblings in the Moot which made him prematurely dissolve it and begin arresting leading members. ...
MT
Rebellion Sourcebook, "Windhook’s story was transcribed privately by a computer information agency at lnarli in Corridor Sector." Windhook fled Capital in an "...Imperiallines-type TJ cruiser bound for the Imperial border ... Within a few weeks, he had switched ships four times and used every trick that Naval Intelligence had ever taught him, hoping against hope that a young naval lieutenant could hold out against what must be the whole Imperium chasing him."
Inarli's 64 parsecs off - minimum 7 weeks, likely longer depending on what other ships he took and what routes they followed. Return communication's at least 7 weeks at J6 if the Admiralty's using fleet couriers to distribute such allegations, more like 16+ if the X-boat network's carrying the story. We're looking at around 6 months before the allegation gets to Capital. At that point it's an allegation from a distant border world made by a man suspected of being an agent of Dulinor.
I do not like Lucan - no surprise, he's not written to be likable. I do consider him to be capable of murdering his brother. However, I do not think the word of a fugitive implicated in the murder, six months after the deed, is going to carry much weight outside of the Imperial equivalent of the
National Enquirer and perhaps the far future equivalent of Oliver Stone. The crazier Lucan got, the more weight the allegation would carry, but early on in the business it would have essentially no impact.
Now, let's look at the idea of the Imperium as some sort of feudal equivalent of the U.S. under the Articles of Confederation. (Okay, that overstates it - a bit - but you get my point.)
The Third Imperium is, as of the assassination 1116 years old. Over that period, it has expanded from its Sylean Federation roots to a polity encompassing almost 9000 worlds in 28 sectors with a population in the vicinity of 15 trillion souls. It is famously conservative.
It is also famously resistant to challenges to its authority.
Martin III crushed the Ilelish Revolt, a revolt that at its height covered six sectors, in a 17 year campaign that ended with the equatorial regions of Ilelish being reduced to desert as an abject example of the price of rebellion. The Imperium did not fragment then. Odd quarters of it did not decide they would put their own candidates forward as claimants to the Imperial throne while the Imperial administration was actively engaged in war. It endured - and Ilelish and her allies lost.
Cleon IV rose to power by assassinating Nicholle, endured 80 years as emperor before being assassinated himself and triggering another 67 years of accession by murder - including a delightful 16 year period in which the fleet admirals one after another used the forces at their disposal to elevate themselves and overthrow their opponents at meson-point, so to speak. The Imperium did not fragment then. Odd quarters of it did not decide they would put their own candidates forward as claimants to the Imperial throne while the Imperial administration was actively engaged in war - well, the fleet admirals did, but by all accounts the Imperium as a whole went on about its business while the admirals happily shot at each other. The Imperium endured - and the admirals killed each other until one had the good sense to try another way.
And on that subject, there is the Spinward Marches, home to the infamous Olav who rode into Core at the head of a fleet to start the cycle of emperors-at-mesonpoint. Dissatisfaction with the Emperor did not prompt them to leave the Imperium and go do their own thing at the end of the First Frontier War, though the tactical situation in Corridor and the strength of a fleet able to march on Core suggested they could have done just that. They remained with the Imperium, and Olav took his fleet to Core to go "fix" things. Nor did they decide to do their own thing when the Third Frontier War gave them cause to do just that. No, that war instead brought the abdication of an emperor. Which brought us -
- Gavin, who mounted war against the former Solomani Autonomous Region when it sought independence in the aftermath of the abdication of Styryx. It was by all accounts a bloody and expensive war: the Solomani Sphere encompassed most of 9 sectors, the effort ran 12 years with conquest of about a quarter of that territory, ending in the conquest of the Solomani homeworld and an exhausted Imperium and Solomani calling an armistice that lasted over a century. The Imperium did not fragment then. Odd quarters of it did not decide they would put their own candidates forward as claimants to the Imperial throne while the Imperial administration was actively engaged in war. It endured - and the Solomani spent the next century looking at their occupied homeworld from across a hostile border.
This is a conservative Imperium which has hung together for over a thousand years, despite incidents that could have torn it apart - incidents little different from the assassination of Strephon and the rebellion of Ilelish, when it comes down to it. Strephon was, by most accounts, not a bad emperor. Dulinor is the first hint we have that there's a serious grievance to be had. Tell me that he had been lousy, controversial, that his nobles were dissatisfied and sick of him and his line, and maybe the MegaTrav scenario makes sense. Give me an Imperium of a thousand years and more with a fairly well-liked Emperor suddenly slain in his throne room and one clearly eligible successor taking the throne, and I say it is absurd for the Imperium to fracture as it did within no more than a year of his death.
... Or the preconceptions of TL15 cloning based on Dolly the sheep isn't quite right and clone is just the closest word we have for it. ...
Or you should do some research. Fingerprints and retinal patterns are unique, even among identical twins. Your suggestion implies that an emperor who wanted someone to play-act him when he was away from court went to the trouble to biologically engineer a double so similar that the Emperor himself was at risk of being replaced by it - presumably just in case some upstart courtier demanded his lord subject himself to a retina scan to prove himself. Coming up with an excuse to need a double that exact stretches the suspension of disbelief like taffy in a taffy puller.
Allow me to paint a more realistic picture.
Dulinor assassinates Strephon - or maybe the Strephon clone. His coup running into trouble, he flees while his men buy a couple of days to cover his escape. His J6 ship books it for Dlan. Supposedly he arrives ahead of word - but word is travelling at J6 too, at least for the navy that's being ordered to arrest him. However, that's a minor point: no great difficulty to evade capture until he can reach Ilelish space, where loyal allies will stop the word from going further and see that only his version goes farther. He has the edge in timing - word is going out to his fleets and they are preparing even as he arrives at Dlan. And, he has no choice: take the gamble, or wait for Imperial Marines from Core to show up and arrest him.
However
Ilelish has 16 fleets, plus the reserve fleets. Zarushagar, also part of the Domain of Ilelish, has another 15 plus reserve fleets, though the first battle map in Survival Margin seems to imply that some of the more distant fleets on Zarushagar's trailing edge maybe opted to remain loyal to the Imperium. Ties of friendship and personal loyalties might deliver another 3 or 4 fleets from Gushemege and as many from Dagudashaag, but this is uncertain, and at any event some force must be left in place to stop potential flanking attacks from Gushemege and Masilia. Verge doesn't seem interested in joining the fight. So, roughly 25 to 30 first-rate fleets are heading toward Core, with a few more covering a potential Massilia flank and a few covering a potential Gushemege flank, and the colonial - er, reserve - fleets in, well, reserve. Best guess is Dulinor's fleets could have been engaging Imperial assets by the end of 1116 to early 1117 - which is consistent with Lucan getting word and panicking around 90-1117.
Arrayed against Dulinor are the 16 fleets of Core and the 4 fleets of Capital, the 12-ish fleets of Dagudashaag that remain loyal - those two sectors alone with sufficient strength to stop Dulinor once they get organized - the 10-ish loyal (until Strephon pops up) fleets of Gushemege, and the 16 fleets of Massilia plus maybe what appear to be a few Zarushagar fleets on the rimward edge that remain loyal. In other words, Dulinor is opposed by about twice his strength. As of 90-1117 there is absolutely no reason to expect the fleets of Massilia and Gushemege to not respond when the legitimate heir of their murdered Emperor calls them to war against the man who murdered him. Takes about a month for word to reach them from Capital, figure another month for them to issue orders to their fleets and those fleets to start moving. Around mid-1117 - before Strephon ever makes his announcement - Dulinor is faced with his thrust on Capital being stalled somewhere in Dagudashaag while the combined forces of two sectors thrust in at either flank, opposed by a handful of main fleets and his reserve fleets. Note that under this timeline, by the time Strephon speaks on the balcony at Usdiki, Gushemege's fleets are already on the Ilelish border fighting the few rebel main fleets screening that border.