On challenges that come with an item cost (eg 100 souls), if I use a second chance, fail the first attempt and then simply give up rather than trying again, will I retain the items, or still have to pay the cost?[li]
Typically, I believe the item loss is a consequence of losing (and sometimes succeeding) the challenge, which you do not do if you back out. Things that are consumed on-attempt, like actions, are lost if you leave, so the quickest way to find out for sure would probably be to attempt the second chance, fail, and before deciding whether to back out or try again, refresh the page and go check if the items have been removed.
The items in question cost over 400 echoes and I don’t have any other easily failable challenges, so I’d like to know for sure before I risk them!
You’re fine. I used the Second Chance trick to get an Impossible Theorem, and the Searing Enigmas weren’t consumed when I "failed" a couple times.