B
Another thought to consider:
Maybe not buy the acoustic drum set. If you're looking to isolate sound, a good idea would be not to create sound. You already have the edrums and there are some very good drum VSTi's on the market that can be triggered from your edrums. Look at EZDrummer, Steven Slate Drums, Addictive Drums, etc. These programs run inside your computer as part of a DAW program. If you're going analog, then you might not be able to use them, but if you're going digital, you'll most likely get a better sound than the edrum sound module can produce. And you won't make nearly as much noise to disturb the family. And you won't spend as much time, energy or money on trying to isolate the room only to find you still have noise leaks.
Again, just a thought.
Yes, the stairs are split (sorry, not sure how to draw that). unfortunately, I do need that exterior door. I wish i didn't because it would be nice to have space that the new hallway will take.