Hi there! Just finished bingeing. Good quest.
[X] Seal the damn room; then we have two rather hulking 11 year olds plus a Malfoy in front of a sealed door and can get the rat at our leisure? (Also doors are much easier to HIT than running rats).
This sounds good to me in the absence of DC estimates.
On the topic of build... could I convince people to go for a stealth and infowar build? Extend our mage sight and detect evil and so on as far as possible, combine with invisibility and teleportation effects. I've seen a lot of gamer quests go straight for magic blastiness or punching punching punching, and that gets boring after a while. I've seen a few non-quest gamer fics turn observe into high-end sensory abilities - The Games We Play is the biggest example - but even then the additional information is always just used to choose what to blow up. I'd greatly enjoy a Gamer that bypasses the whole blowing-things-up route and leans on invisibility, teleportation, illusions, sleight of hand, mind-reading, sneak attacks, assassination, and information warfare. And if any setting supports that it's Harry Potter. AFAIK wizards have few, if any, effective detection abilities. They have few, if any, effective passive defenses. All defense is active. They have few, if any, attacks that aren't actively triggered and targeted. They don't expect teleport spam, tactical mind control (command, suggestion, charm person), people intercepting their owls, or anything even remotely like programmed illusion or fog. We could make Voldemort completely impotent just by teleporting around while invisible and not using attacks that reveal our location.