My understanding was that the Vaygr were not the shambles of the former Taiidan Empire… they merely absorbed those remains. The Taiidan just linked up with Makaan to exploit an opportunity to be part of the Hiigarans’ downfall, and they only bumped into each other in the first place because Makaan was invading the coreward regions. The Vaygr have been roaming the Eastern Fringe of the galaxy for presumably quite a long time… since they have their own lawless region to roam, the Empire not falling probably would not have any impact on their development. Like, their technology is totally different from galactic standard (no ions - their only beam weapon technology is in miniature, exactly the opposite of everyone else). As for the Taiidan finding the 3rd Core instead, remember that the Empire was poised to split at the time of the original Kushan return. Without the depopulation of Kharak, there would have been no precipitating factor to begin the Taiidan Rebellion, but the cohesion of the Empire probably would have remained uneasy… it is doubtful they would have expanded much if at all in those 115 extra years.
That’s possible, but the Gaalsien were sufficiently advanced to be thriving in the deep desert while the more fertile North was already starting to choke. If things got very much worse over those 115 extra years, I can see the Gaalsi abandoning their holdings and moving toward the poles, but I expect with their survival techniques and technology they would have been able to sustain life there for an extra century, given the much worse conditions they were abiding in at the time of DoK.
They could do that, but they’d be missing an opportunity. The Vaygr and Gaalsien have such fortuitously symbiotic beliefs. Gaalsien are waiting for some sign from Sajuuk that the Kushan have humbled themselves and repented sufficiently to reascend, and Makaan is eager to proclaim himself the Sajuuk-Khar. Here he finds an opportunity to lend himself considerable credibility as the chosen one by not only taking another step to fulfilling his own destiny, but also allowing the Kushan to fulfill their destiny in the name of the same god. Like, all he would have to do is invite the Gaalsien to join him as the proclaimed Sajuuk-Khar and they would be all-in. He’d have a new and incredibly zealous fighting force in no time to aid in his ongoing crusade to attain the last Core and conquer the galaxy. And what would he have to give the Gaaslien-led Kushan in return? At most, Hiigara - the paradise they were exiled from. And what is Hiigara to Makaan? Nothing. The Vaygr are space nomads. And he would have the rest of the galaxy anyway. He’d be revered by the Kushan for all time as the Sajuuk-Khar, which for all intents and purposes he would have proven himself to be.