making converted attic into full height (roof tear off)
There are lots of questions, I'm not sure if I got them all

1: your first level studs will be fine. It's good practice to align joists and 2nd level studs over the first level but is usually not necessary

2: There will be block that will need to be installed between the floor joists and the floor sheathing will be nailed over that. This is installed for several reasons including for a load path between the floors for the lateral forces, and to stop the joists from 'rolling' over. There is no requirement that the blocking be at the exterior face of the wall, and in this case I would put it at the interior face so that more of the floor joist depth is engaged to resist the rolling

3: there should be no cutting to fit the bottom plate. The second level sheathing is placed over the joists and blocking to the exterior face of the exterior wall studs. Your wall is built in this 'deck' just as your first level walls

Other items to consider:


you might have height restrictions which will not allow you to do this remodel
you will need to decide how to deal with the new upper walls and the brick below. Brick is heavy, and while your foundation may be adequate for the upper level, it might not be for another level of brick
For a change such as this, there will be a requirement to bring your lateral system up to code. This may, and is likely, to require work in the first level walls. Which means, opening them up...