> clicking back on a slide-show bringing you to the overview page

That behavior is expected in exactly one case (assuming slides, not the whole presentation, are modeled as a page each): If I navigated to that specific slide from the overview.

In any other scenario, this behavior amounts to breaking my back button, and I'll probably never visit the site again if I have that choice.

Opt in features are a great way to increase user frustration and confusion. See the whole new geolocation API they had to make for browsers since people would perma-deny it reflexively and then complain that geolocation features weren't working.