About stealth, I think (excuse my lack of modding knowledge) there's a way to trigger people attacking you.
I mean "belligerent drunk" in Native taverns, only when you approach him, he starts attacking you.
Also in some mods, when you shoot (1866, WWII China) in towncenter scenes, guards start attacking you.
So IMO the system would be like : guards are marching from one point to another in the scene. You could use stealth staying at bay frome the guards, or if you're too close to them and they remark you, you can kill them all or avoid them to clear the mission.
EDIT : Didn't see your post Hanakoganei
But you did give me an idea of a script I might be able to put together. It'll be like the sneak into town, advanced version. I'll borrow the Belligerent Drunk's script. He doesn't check line of sight but he does check proximity. So if you somehow manage to stay away from him, he won't ever try to attack you. Now we apply that to town guards. Basically if you can stay far from guards and stuff you're being stealthy. It's the best we can do with the current engine.