Snape nodded. "I haven't seen that spell before, myself." And an interesting experience it was. Now that I've cracked it, I should be able to cast it. "I suspect that as Riddle aged he shelved such relatively subtle measures, in favour of more extraverted curses such as Imperius. More impressive for the minions," he sneered.
"As for your guest," Snape winced and shook his head. "Riddle didn't design the Invisiperio to be counteracted, ever. He's never cared what damage his curses do to his toys. I believe that when it broke, the shards - the mental conflict between the truth and the memories Riddle implanted - injured the man's sanity. The damage is probably as irreparable as prolonged Cruciatus - but I'm not a Mediwizard, and of course," Snape finished ironically, "you're welcome to seek a second opinion."
"You'll need to tell Riddle that you took your time torturing 'your daughter's killer', and that as far as you're concerned that favour compensates for her loss. We'll be able to deduce more of his motives when he contacts you with further orders."
"As for the information leak, in the first instance, is there any way you could rotate job duties within the Department quickly, especially moving people formerly in higher-security areas to lower ones and vice versa? In longer term, seconding people to other departments and bringing in new blood would also help dilute the impact of the information."
Snape hmmed. "Well, he did have her duelling with and injuring him in the memory - which never actually happened in the attack itself - so presumably he made that change in the hope that would give enough of an accounting of her combat skills.
You know her well enough to know she wouldn't scream in fear like that. Riddle doesn't know that at all, so no, he couldn't realise that part of the memory was wrong. I'm sure that - as long as he never finds out you and I have been in contact - he will feel secure that this deception has succeeded."