Would six months be long enough to have all the masts and wires up, signal immunization done, etc?
The six month closure wouldn't be exclusively for electrification - that's all work which can be done at nights, during normal possessions, but it's only 24 single track kilometres to wire plus wiring in to the existing network and taking the feed where needed via the existing feeding arrangements.
24stk should be do-able in a month's worth of overnight possessions, getting 1km of wiring done overnight isn't unduly difficult, so the actual wiring arrangements would be easily done within probably a week to 10 days of a full closure.
Masts etc - rough count would be 500ish, again, a rate of 10 to 20 piles per night depending on plant and ground conditions shouldn't be an issue, so having full closure for 6 months, would get that part done within a couple of weeks, maybe three. It'll depend on twin track cantilevers, portals etc at junctions, which will need to be done in a specific order due to the need to electrically isolate existing electrified routes (some of which will be isolated anyway for CrossRail or other routine maintenance and renewals).
There's also platform extensions, gauge clearance works and signalling immunisation (though most should be immunised as the route has been plotted for infill electrification for a long, long time, and parts of the route are already electrified).
So yes, six months will be plenty of time to do what's needed, but there's ongoing evening and weekend work at the moment, so what will be done during a six month closure will be less than everything that's needed.
Surprised Rye House is considered too far north. 
