For #1, it really is dependant on the individual's system. The more the game has to load, the more potential for lag there will be, as such, the more powerful the person's computer, the more it can handle before starting to show signs of overload.
General rule is to keep it as simple as possible. Do you REALLY need every light source to cast dynamic shadows? Probably not. Do you NEED to cover everything in ivy and flora? Again, probably not. Keep things simple and to the point and cut where you can. Havoked clutter is another biggie, as they need to invoke the physics engine. The more physics the game needs to process, the more potential for lag. Same with scripts. The more you have running, the more CPU time is needed, thus, like a parrot, the more potential for lag.
That said, you might look into render boxes. (Is that their technical name?) Basically they are boxes you use to encase parts of your interior levels and are connected via "portals" (Don't worry, no loading screens) This way the game only renders (though still loads) what is visible inside the boxes. There is a an official Bethesda tutorial for this on Youtube.
Also something to remember. Light sources are heavy on the rendering, and there is a hard limit of 4 omni lights (No shadows) and 3 shadow casting lights. If there is more then that on screen (Even if they are behind objects and you aren't using render boxes, they will start to turn on and/or off.