Just confirming what has already been said above. The Di2 will most likely need to be partially external, at least for the junction box as there really isn't a place to slide an internal one into the frame.
Also, as for Stages, the standard road models will not work. I ended up picking up a Sram X7 mountain bike Stages and paired that with a Force 1 drive-side crank. The spacing on the X7 gave enough clearance, and I've never noticed any issues as far as having the Q-factor being slightly different on the non-drive side.
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