Even if we set the Witchbombing risks aside for the moment (and I think they're serious enough that messing around is still an awful idea), trying to make a deal with Airi with Yuuri's seed still has a huge chance of provoking her.
It's making a deal with a dangerously unstable individual that we have no guarantee of being able to follow through on, given that we don't actually have dewitching yet.
I think you're making some leaps of logic.
I expect the Seed to be at Incubator world, so this would be just confirming the Seed is out of reach.
If it is in reach, however, I want (us or Kazumi) to hang onto it in case it becomes useful as a bargaining chip in the future; after Airi's been disabused of her crazy, most likely.
[X] Be nice, tactful, and supportive. Brinapilot best order:
[X] Refuse Kazumi's offer. You can't leave your friends in trouble. You want to try and help Hijiri and Airi, too.
[X] Kazumi, privately: Try to find whether Arzt Kochen's Seed is still around without raising alarm bells.
[X] Hijiri, privately, after she's done with Niko: Make the case that Airi needs help; encourage Hijiri to be honest, for her own good; tactfully bring up Airi's allegations if possible.
[X] Try to talk about conditions for Hijiri's future release, as an olive branch towards Hijiri, but mind both Kannas' state of mind.
[X] Propose ideas for finding Airi quicker; encourage others.
-[X] E.g.: Sensing Booster + Ribbon Range extension; Sayaka/Kazumi clones; everyone can spam magic; Magic booster could help.
-[X] Could Hijiri help?
[X] Share all relevant details with Mami and Kazumi. Inform Homura, too.