Why?
Extending an extra tube line into an area already served by the Piccadilly, Bakerloo (plus Overground) and (on the periphery) the Metropolitan line makes no sense whatsover.
The point is that the stations are near to useless for access to London for the reasons you state - so their only use is for local journeys, which means you need to serve Neasden with a frequent service, which you can't do with a NR service as there a) isn't the line capacity and b) there isn't the capacity at Marylebone due to the lack of platforms.
For the most part, the Bakerloo duplicates the Central London connectivity that a Jubilee extension would provide anyway. And you-ve just stuffed the capability of the Jubillee Line to clear passengers from events at Wembley too, by halving the service between Wembleys Park and Stadium.
The line wouldn't reduce capacity to Wembley at all - it would be serving Wembley Stadium station, effectively splitting the crowds between two stations, so would dilute the crowds far more effectively than is the case at present. The point you seem to have missed is that the branch would primarily be served by trains that currently turn back at West Hampstead and Willesden Green. Ideally, those turning back at Wembley Park would also be able to potentially serve it, giving a general 50/50 split between the Stanmore (Wembley Park) and Sudbury (Wembley Stadium) service.
As I stated originally, the problem TfL have is that Jubilee Line's full capacity is now needed as far as Wembley (making the turnbacks at West Hampstead and Willesden green effectively useless), but this capacity is not needed in the slightest on the Stanmore branch, and the Wembley Park turnback can't handle turning back the entire Jubilee service on its own (which is why Stanmore got its 3rd platform). Hence their proposal to take over the Uxbridge branch to try and use this capacity more usefully.
In any case, the wide Chiltern Route formation at this point would be exceedingly useful for masts should the line ever be electrified.
Possibly, but given the footprint required for a mast, I'd wager that any space that could be used for a track is more useful with one of those on it rather than a mast which could simply be placed further back from the lines.