I still don't understand why capturing an Eshin skaven sorcerer is even a subject of discussion.
It involves:
1) neutralizing its magic during containment, a magic specifically designed for infiltration and exfiltration.
2) convincing an Eshin to betray one of the most guarded secrets of his own clan, completely, utterly not the same as a disgruntled Moulder chieftain.
3) keeping for an indeterminate length of time someone that is likely as competent as an Eshin Assassin in our dungeon, in our own fucking home.
4) hoping that we manage to separate the bullshit he will say from the maybe truth and technical truth in order to have an accurate understanding of its magic system (reminder: he is a master at intrigue and lying).
This is a bad idea. As in parlaying with a dragon in the middle of a battle seems like the pinnacle of common sense in comparison. Just write off the magic portion of the Skaven dictionary or let someone who has more time or is more insane do it using the basis we already wrote. Make the magic portion of Queekish be someone else's problem or hope to find magical papers in the ruins of Yar. We have already enough shit to do without putting up with that kind of unending headache. Good enough would already be a ground-breaking achievement.