Is this riddle missible?

[Cauchy-Schwarz]

I just asked this on the PS3 forums, but I thought I'd ask here too... I'm looking to solve the riddle "Can you find this cool couple before the romance thaws?"

I know exactly what I need to do and the guide on gamesradar confirms it, but my problem is...

Spoiler! (click here to reveal)

that Mr. Freeze doesn't seem to want to go visit his special lady.

Does anybody have any idea why? And what needs to be done to fix this?

It may be worth noting that I've completed the game, but that shouldn't matter, right?

[Seiryu]

It's not missable.

Spoiler! (click here to reveal)

Once you complete everything story related with Mr. Freeze he will ask you to find Nora, and it is a side mission. After you have found Nora you have to return to Mr. Freeze to tell him where she is, then he will go to her, which completes the side mission. From that point on he will always be standing next to her in the same place where you found her originally.

So, in short, once you complete everything required to make that happen, the riddle solution will be there and stays there permanently for the remainder of the game and during free-roam.