I'd not this isn't (mainly) about Starke's interrogation of Mathilde. For one he'd have better sources than asking her to her face what she thinks and giving her an open license to lie.
This is about the public face the College present about it.
[ ] Piety
You converted the Wurtbad Watch to Ranald because you worship Ranald, and it would have given you a better hold over the organization. Simple and true, but it would mean revealing your faith.
This essentially makes us a public face of Ranald, doign heroism in the shadows.
Not really something a GOOD Ranaldite is supposed to do, but our piety is fairly well known.
Downside is that it entangles us into faith issues, for instance, it would be very awkward to go to Bretonnia to recruit for the Waystone Project while they pretend we don't worship Ranald, while we openly worship Ranald. Not really a showstopper by any standards, but you get the good with the bad.
[ ] Grief
You acted rashly after the death of Abelhelm, a death that Sigmar could have prevented, and acted out against a largely Sigmarite institution under your control. While true and understandable, it does hint towards the extent of your rather complicated feelings towards your former liege, and you might not want those hints available for cross-referencing if the Underwear Incident ever comes to light.
This spins the public story as Mathilde's strong loyalty to her employer, despite said employer being traditionally opposed to wizards. Our resume backs it up, we worked for a Witch Hunter, then for a dwarf, serving both with impeccable loyalty.
Its not a bad brand to have, but on the flip side, it allows for less proper rumors, because we aren't the impartial sword of the Grey, but can be swayed by personal attachments(we can of course be swayed, but if we advertise it, it opens us up to more people actively befriending with the expectation of returns.
[ ] Trauma
You stood over your dying liege and fought against an endless tide of undead, waiting for reinforcements that almost never came. You took command when nobody else would and gave orders that led to the deaths of thousands. You sentenced a Necromancer to death and smiled as you watched him burn alive. In the aftermath of all that, you were not at your best. That's why you mishandled the situation.
This focuses on Mathilde's sheer youth. And to be honest she was for all purposes a kid back then. In over her depth. The spin would be on Mathilde being a genius, and young, but while she's no longer very young, it remains a reminder of her relative youth for her rank. Look to Mathilde for the brilliant on the spot decisions, not for well grounded mature decisionmaking.
It also shows a more human side of course, less illusion of infallibility.
[ ] Skaven
Sure, you didn't know about them then, but Reiner doesn't know that. Say that you had plans to turn the Watch into an instrument for defending against the Skaven. As part of the Conspiracy of Silence, Sigmarite creed denies the existence of the Skaven, so it was necessary to convert the Watch to some other God, and the Night Prowler was the best fit for a subterranean war.
The "Just As Planned" party line. Its all a scheme from 20 years ago fresh out of College.
Mathilde is in control and always have been.
Of course, this is a bit porous if you poked deeper(which Starke could, but whether he'd bother varies), certainly Skaven AREN'T involved in Stirland
But the party line is that she was a wise old sage from the moment she graduated and everything is part of her master plan.
[ ] Nothing
Give a non-answer. You don't fully remember the details, but you made the best decision you could with the information you had available to you at the time. With the benefit of hindsight you would have done things differently.
This is the Officially Redacted approach. Just put up a wall of Grey College No Comment, and it'd be buried to the public.
[ ] Leave
Reiner has been very clear that you can just leave. Do so.
"Eh, I don't really give a shit, do whatever"