As for the skins, cannot disagree with that. Until they are used (or if they are won through character rewards), they should be tradeable.
I have done the instigator at least 20 times and have not received a crate from him nor the chest. I have been averaging about 2-3 an hour farming karka or skelk outside of the instigators. Which champs are you talking about (outside of the Karka you already mentioned)
Edit - FYI my MF with all buffs is sitting at 368