The stuff you can help yourself to is in a little grotto that was previously blocked by plantlife. It's just behind Derreth & Baelen's ox in the Myconid Village.
I must have missed it, didn't spend too much time looking around there before going back to the Slavers.
I'm afraid I've never even tried to help the slavers get the boots back, so can't comment on that bit, but it does sound like you've found a bug.
EDIT: Unless you spoke to Gekh with a different character the second time around, which would be easy to do especially if the first dialogue with him autotriggered - as I think it might - and caught someone other than your main character? The game doesn't handle that situation well, and the new character usually just gets the whole conversation again, with no acknowledgement they were right there the last time. Did you try just giving the same responses as the first time and then talking to again with the same character? It's possible that could move things along.
Your edit suggestions sounds like a possibility, I did try giving the same responses but nothing positive came of that, regardless even if it did give a positive result though anyone having to do that means its still a bug.
Fingers crossed Larian do pop in to this forum and take notes of what needs to be looked at.
