If you're considering starting your own thread on your alternative Battersea proposal, I'd certainly be interested, as I'm sure others would be.
The NLE Battersea branch will be connected to the outer lines at Kennington, which means trains to/from Battersea will always use the Charing Cross branch. The run-around loop between the outer platforms will remain in place for the sake operational convenience with terminating trains.
I refer here to the TfL "Proposed route and key sites" map:
https://consultations.tfl.gov.uk/tube/nle/consult_view
The reason why the Northern Line will (eventually) be split into two separate Edgware-(Charing Cross)-Battersea and High Barnet-(Bank)-Morden lines is because the two depots are located at Golders Green and Morden. If the lines were to be split through Edgware-(Bank)-Morden and High Barnet-(Charing Cross)-Battersea, the former line would (as things stand) sport two depots and the second none.
Is it the case then that the Jubilee Line also requires additional units in the future? IIRC the 96/95-type TSs cannot be built any more. This will mean that some consideration is required as to which lines deserve a new fleet when, and whether displaced units can be cascaded to another line. The two Alstom builds may yet find a bit more life remaining in them.
As I understood it, one of the two present Northern Line 'branches' will receive a new train fleet, with the displaced 95TS allocation to be distributed between the other Northern 'branch' and the Jubilee line - though in the latter case, it would require a unit or two to be disbanded in order to make up the rest to seven cars.
On the other hand, would 95/96TS fleet standardisation on the Jubilee Line be economical, with regards to their remaining lifetime. Aside from the unit formation and traction packages, what other differences remain?