MC Cable through double 2x4 top plate
I would be placing a junction box in the attic and only running 1 line down to each receptacle.
Running up and down is a waste of wire and if the walls are not open a lot more work fishing metal clad can be a pain in the back side especially if there is insulation.
Having a 4x4 box makes a great junction box get a deep box with all 1/2 or 1/2 -3/4 TKO and you will only need 1 box for most rooms.
When I usually do this I many times add a raised single duplex cover and add a convince outlet it can be handy to have if there are no outlets up there.
With balloon framing you need to reduce the amount of damage (drilling) I don’t work with balloon as often as it is somewhat rare but you need to be aware additional holes will compromise the structure.
Make sure to use the insulating bushings that came with the MC inspectors look for this if your roll doesn't have a bag get some they are only a couple of $ for 50 or 100 I cant remember how small the bags come.