I am playing a solo 3 character co-op game and just about to do the showdown.
What happens in the event all 3 of my characters get knocked out? This ends the showdown but does that mean its game over? Should i be aiming to get one character out before this happens? Or can i recover all 3 characters in the Mystery phase (which I assume I would play after the showdown ends).
You lose a co-op game if all heroes are knocked out. See the rule at the bottom right of page 21. In one of the expansions, there is an exception to this rule for games with only 1 hero (and maybe only 2 heroes, but I can't remember for sure).
What about when an evil elder 'cannot be targeted' during a showdown. What happens in this case? How do you kill them? When you do kill them do they move the shadow track 2 spaces or is it only elders who join you in the hunting party that move the shadow track?
If they have the "Darkest Secret", then the evil elder is basically considered to be the villain, so you can't target them separately. They will still die when the villain is about to take his last wound. In that case, the evil elder takes it instead, and the villain is still alive.
Also, evil elders are no longer considered town elders, so normal town elder rules don't apply. That means you don't move the shadow track when they die. Confirmation from the designer.