It’s still possible the kid’s not Joker. Previews mentioned a new criminal “The Red Hood”.
Joker, so far, seems to be a kind of spirit of Arkham that has yet to settle on a host body, if the laughter when the fake nurse got her face bloodied was more than just a stylistic choice. I think “creepy guy in police interview room” is just more fan service, like the bad comedian in Fish’s club; the show sometimes takes the easy route and draws from the successes of past works instead of making its own. The alternate histories from the comics suggest that Gotham will always make a Joker; the show can stretch this out for years.
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Gaultheria's fanvids: http://youtube.com/sagethrasher