Sorry but that is mostly non-canon / made up.
The Aslan ambassador tries to save Strephon and dies, yes. But the Aslan do not blame the 3I - they blame Dulinor and at best his faction
The Ithahei do not close the rift crossing on their own. The home clans do a "last stand at Camerone" thing and order them not to return/destroy all that comes after the messenger
The Domain of Deneb is not effected by the imperial cival war. Norris conveniently err finds another Imperial Warrent, declares himself ArchDuke, takes his toys and makes up his own lil playground
"Mostly?"
The migration of the already trans-Rift Aslan is a direct consequence of the assassination and subsequent fracturing of the Imperium. The new ihatei from the central Heirate are not, as they were launched earlier, but their redirection into Imperial territory when they arrive definitely is.
-Rebellion Sourcebook, GDW, 1988, pages 58-61
The ihatei did not "close" the trans-Rift path. The central Hierate made it clear at a cultural level that ihatei sent that way don't come back, but the route wasn't closed, as such.
I will cede the idea that the Aslan Ambassador's death being a strong cause may be interpreted as an unreliable narrator account, something which MegaTraveller engaged in sporadically, but Hard Times maps do show conflict and a fall into wilds for much of Daibei, including areas the Solomani should have no interest in. That can quite reasonably be blamed on Aslan incursions.
Your last statement indicates a disdain for previously published Canon, so discussing the details any further is probably not worth your time or mine.
Should also point out that we are talking about events in the future.
And the future is not set.
If Mongoose wants to foster another timeline split that's on you, but there are plenty of stories to tell in the Rebellion period
without changing the big story. Literally ten thousand worlds worth, over a twelve year period. The Fifth Frontier War is smaller, but similar; there is no need to change the outcome to produce more material.