I don't know what your house is like, but in my house I could shoot just about any angle I might want from my bedroom door (straight out, down the hall towards stairs, at the head of the stairs) or from the top of the stairs downwards without having a bedroom under the muzzle. Here's some typical three bedroom semi layouts:
As you can see, all three bedrooms are clear with regards other bedrooms for shooting straight out the door or at the stairs. Equally, you can shoot down the stairs. In the first layout, only bedroom 2 has to worry about windows (the one at the top of the stairs). In the second, bedroom 3 is pretty safe but 1 and 2 need to think about it. But the window isn't necessarily a problem - it depends what's outside. For example, from the master bedroom in my house, shooting at the landing window would send pellets into the side of my neighbour's brick house. For penetration to be an issue you'd need 1) spots you're likely to be shooting from and to, to match up with a window and 2) for that window to match up with an opening on a neighbouring building and 3) that's a window to a bedroom. And even if that is the case which I doubt it is for most people, will birdshot be stopped by two windows? I honestly don't know so perhaps this calls for some testing!
If you do have a major issue with firing from or at the doorway, there is the option to shoot across it instead. If you have adequate room to position yourself on one side of the door, and there is a relatively safe area to shoot at on the opposite side, then you can wait there. For example, in my old house, firing straight on at my bedroom door would have sent rounds into the bedroom opposite, but firing across the door would put rounds into the airing cupboard.
As for lethality, here are some real world birdshot wounds at ranges estimated from 3, 7 and 15 yards:
(SERIOUSLY GRAPHIC)

******************************

*****************************

***************************

**************************

*************************

************************

***********************

**********************

*********************

********************

*******************

******************

*****************

****************

***************

**************

*************

************

***********

**********

*********

********

*******

******

*****

****

***

**

*

***
**
*
You can see how shallow these wounds are. None are physically man-stopping. You might fall down because of the pain or psychological shock of being shot, but this is not very common in gunshot victims who are aware they are being shot at (as an attacker would probably be).
This is a coyote taken with buckshot at range:
He was shot on his right side, the pellets travelling completely through the torso and logding in the ribcage on his left side (top). One pellet shattered his leg completely (bottom). Here you can see remote wounding done to his liver:
And here (skinned and decapitated) you can see the enormous internal bleeding:
