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It's interesting how much Saren Manor  is designed like a sneak challenge. There is a backdoor allowing to go through a longer route, but potentially catching Toris Saren unawared. Here is the first part of this route. Enter throught the door, evade the attention of Arns Saren, by passing over the railing and go to the next location through the trapdoor. There are even chairs near the railing, from which it's easy to jump on it.

However, this is not pursued. Arns Saren doesn't actively try to detect you and there is no reason not to pass through the front door in the first place,
The most annoying is the fact that the railing collision doesn't allow to pass through it. This ruins the whole interesting setting, Thankfully this is very much a fixable problem even without editing meshes.