Punishments

A few questions:
Access to the Redcap's corpse to ask its spirit questions? Or was it buried in a churchyard?
Was this Redcap young and had few to no magical items to aid in escape? (If he had a cloak of teleportation, not getting away is his fault, as example).
I'm curious as to what kind of faeries they were.

As far as,

I always was under the impression that faeries probably won't kill anyone who's following the story properly (sovereign wards, et cetera.) But a lot of faeries eat people or kill people to keep the story 'real'. I could also see a faerie noble going to the Redcap and saying, 'You have wounded us, give me your daughter as repayment' and the Redcap justifiably fought back.

The angle that the Redcap-is-at-fault will be useful for adding a plot twist, so thanks to those suggesting it.

The Redcap's spirit was questioned, but I don't have those notes. I don't know if he'll be brought to the hearing.

I'd judge that the maga had committed no crime in saying what she did, though she deserves informal censure for being an idiot. The Redcap was a bigger idiot, interfering with mundanes and molesting the fay.

That said, if the maga were found at fault, the expected punishment for "molest[ing] the fay, to the ruin of [your] sodales" is a Wizard's March. A thirty years' indenture would be an act of mercy on the Tribunal's part, and the maga would be well-advised not to complain.

Whether House Mercere chooses to pursue vengeance against the faeries in question is up to them; they may well decide that Redcap A was an idiot and let it go, or they might decide to seek vengeance for their errant sodale on general principles.