for what is that flightdeck hump?
Honestly, not quite sure. At a guess, since it was "early days" for aircraft carriers when it was built, maybe they thought it would help with the landings.
Yeah, sounds kinda silly, I know, but I can't think of any logical reason for it. But, like I said earlier, there wasn't much experience with carriers when Hermes was designed and built. I think the only other carriers around at the time were the Vindictive, Furious, Hosho and maybe the Langley, and 3 of those 4 were conversions, not built from the keel up. That doesn't give a whole lot for the designers to work with.