What's the in-story justification for the dementor's presence anyway? I thought it seemed awfully convenient in case Harry decided to demonstrate his Patronus 2.0 but I couldn't figure out how it'd help enough.

I'd forgotten about the potential for ruining others' patronuses, though. That makes a lot more sense, especially considering he'd just reached into his dark side - possibly deeper than he'd ever willingly done before.

My guess: it wouldn't be enough at this point to just demonstrate a superior patronus or tell people about the possibility of ruining it for others. He tells the secret to EVERYONE present, leaving them at his mercy for protection. That gives him plenty of bargaining power and is dramatically Dark to boot. The political implications would be rather interesting, whether the Patroni could be returned by Obliviation or not.