I don't believe you can reach T12 by the stairs, they're the stairs going up from T7, they're just drawn oddly.
To open the door without picking the lock you'd need a key. Krant openly carries the key on his belt, and the section on him says that he passes out most nights either in the kitchen or the dining hall, snoring loudly. You can have your PCs hear him snoring easily from anywhere in T7 or T8, and then anyone capable of decent stealth should have a pretty good chance of stealing the key from him while he's a sleep (basically a -10 penalty on his perception checks), if your PCs are particularly clever and they have any alcohol in their possession they could offer it to Krant as a gift that day, then when he passed out that night you could give him an even bigger penalty on his perception DC.
Conversely, if you want to tip off the PCs to target Krant beyond hearing him snore, you could have Evlar (or a random Acolyte) mention that he heard a rumor that Krant "acquired" a case of whiskey earlier that day and that means they should tread lightly in the morning because he's sure to drink it until he passes out and be hung over the next day.
The heralds and Egarthis also have keys, but it would be much harder to get to them than it is Krant.
Also, the knock spell would have a good chance of success, if one of your PCs has that.