Have you actually looked at Google maps? You need an extra 10-15m width on the existing line throughout (don't forget you need more space to build than to run). At Wimbledon, Apex Rd/Wandsworth Rd/Mile St, Vauxhall, and Tinworth St/Randall Rd, this simply isn't there.
Compulsory purchase of the adjacent blocks and demolition of them would not only be ludicrously expensive it'd also just be pouring a jerry can of petrol on the raging bonfire that is the London Housing market. Any socio-economic benefit you may have achieved for the SW London suburbs is more than offset by booting people out of their brand new flats.
As I mentioned above, my plan would not need any works north/east of the Linford Street curve as I would convert the 3rd Windsor line track into the 6th SWML track, so none of the locations you mention except for Wimbledon would be affected.
Wimbledon Station would need rebuilding, but the new layout would fit entirely on the existing station site, with the trams moving out of the station. The only building that would need to be compulsory purchased and demolished is Wimbledon Bridge House, an office block (and in all honesty getting rid of it would be good for the town centre, and would allow some of the sharp bends on the one way system to be removed).
The fast lines would not need platforms, so the new layout would be:
US |5 6| USF UF DF DSF |7 8| DS RT |9
(where U = Up, D = Down, R = Reversible, F = Fast, S = Slow, SF = Semi Fast, T = Thameslink).
The trams would be moved out of the station into a new terminus directly opposite the station - the line would rise on its existing alignment after Dundonald Road up to the same level as Wimbledon Bridge, and the new tram terminus would be directly above the SWML across the road from the station, exactly where Wimbledon Bridge House is currently, and would have (at least) 4 platforms to allow significant frequency improvements to the Tramlink network (and it would reduce the cost of a new line to Sutton as it would not require a new terminus at South Wimbledon or Colliers Wood).
The approach from Raynes Park already has 6 tracks, 2 being for Thameslink, so they would become the SWML tracks and a new reversible 7th track would be built alongside for Thameslink, which would be entirely on Network Rail land apart from requiring 2 houses at the end of Merton Hall Road to be demolished.
My idea for Wimbledon is certainly is a million times better than some of the plans for CR2 which involve demolishing most of Wimbledon town centre!
And yes, I have looked at Google Maps all the way along the section I propose for 6 tracking, and it is completely possible with minimal compulsory purchasing being necessary - the only properties I think will be affected are a couple of houses on St John's Hill and Strath Terrace that are against the current retaining wall on the southern approach to Clapham Junction, a small pub at Earlsfield, Wimbledon Bridge House and a couple of houses at the end of Merton Hall Road as I mentioned above and maybe part of some back gardens on Toynbee Road during construction.
) showed that extra capacity on the SWML was needed by 2031, and that is extra on top of that assumed to be already provided through Woking flyover plus ETCS+ATO on the fast lines plus some other smaller interventions (every train max length, changes to stopping patterns, etc).
)