Showerthought.
One of ComStars methods of dealing with the Successor States has historically been the inflaming tensions of tensions between the IS powers and false flag attacks are not out of the norm for the organization. After all, if the powers are at war they are not developing, and if they are at war they are destroying infrastructure and technology, and in so doing allowing ComStar to become even stronger. If ComStar were indeed the ones behind the recent terrorist attacks then their objectives were likely as thus:
1.) The retrieval of Helghast technologies.
2.) The sabotage of Helghast research and development.
3.) Pitting Helghan into war with the Celestial Mandate.
If Helghan confronts the Celestial Mandate and does nothing to lessen public calls for reprisal ComStar, presuming this is ComStar, will have achieved one of their likely objectives. While in the short term removing the Mandate lines up with short term interests this doesn't change the fact that this will have been a successful manipulation of the Helghast. It'll make the Helghast a threat to the other Successor States encouraging further conflict and it will establish a precedent of Helghan quickly seeking retribution and help bring the Helghast people down to the mentality of the Inner Sphere. A people easy to anger is one that ComStar and even the other IS powers can easily manipulate. Worst case scenario is someone making use of a similar false flag operations to break apart ORDI.
Blaming the Mandate is bound to bring Helghan closer to war and encourage war among the Helghast. On the flip side, saying nothing won't lessen the calls for war and will weaken the current government. This would result in openings for political rivals of the current administration as well as foreign agents to take advantage of.
For these reasons I believe the authors of the leading plans should consider editing their plans such that rather than immediately blaming the Mandate or staying silent, instead have the Helghast government address its people and state that while they have their suspicions that they have no concrete evidence and that the only thing that they know for certain is that the complexity and sophistication of the operation means that it could not have been pulled off by anyone save the major powers. Have them point out that throughout the history of the Inner Sphere there have been numerous false flag attacks that have spurred on hate and misdirected retaliation that have made trust and cooperation difficult if not impossible throughout the Inner Sphere.
Point out how that has led to near three centuries of devastating warfare throughout the Inner Sphere. Wars so utterly horrifying that it has destroyed planets, resulted in countless war crimes, and even resulted in the loss of technologies that the Helghast would take for granted. Show the people the state of some of Helghast's less off neighbors, independent worlds that have devolved in the wake of these wars with barely any technology and often rife with conflict and injustice. By contrast show how much the Helghast and ORDI have achieved by working together. Emphasize how together they rebuilt a nation devastated by civil war, brought safety and comfort to millions, and even revolutionized the state of interstellar communications in the Periphery.
State that if the Mandate is found to be responsible the Helghast government will act and that the Mandate will get what it deserves, but that no one will manipulate the Helghast people into a war through conjured hate and outrage. Remind the people of the last leaders that did that, of Visari and his ilk, and of the death and devastation that followed in their wake. Remind them of the scars that still remain years after they have long since gone. Emphasize that if Helghan goes to war it will be a just war, not them simply lashing out. Not only because to it is wrong to seek vengeance for vengeance's sake, but that it could well be vengeance wrought upon the wrong people and that in turn would make their actions as great an injustice as what was done to them, and that it would be letting off the true culprits while at the same time handing them the victory that they seek.
Declare that the Helghast and their allies are not the Successor States. That unlike them they will not create and perpetuate cycles of violence but rather the Helghast will strive to become something better regardless of whoever may try to stop them.
Edit:
TLDR: Terror attack fits ComStar MO perfectly. Going to war would be playing right into ComStar's hands. Convince the people that is important to determine who is the true culprit, we don't want some foreign power playing us the way Visari did.