Many of the OP's figures may be open to challenge but I will focus on just one of them: "The steepest incline a train can handle is about 1 in 100." That is nonsense.

Trains coming out of Euston on the present line have been ascending a 1 in 70 gradient since the line was opened in 1837. At first that was considered too steep for the steam locomotives of the day and the bank was rope-worked, but very soon that was abandoned and the steam locomotives took over. Modern trains have much more power available and high-speed trains more still, because they need it for acceleration and to overcome air resistance. Even some places on the existing rail network have gradients much steeper than 1 in 100; for example 1 in 37 between Exeter St Davids and Exeter Central and 1 in 37¾ on the Lickey incline in the West Midlands.

This is not to say that the building of the London end of HS2 won't cause problems. There will be a decade of noise, pollution and disruption for the poor blighters who live near Euston. But the people responsible for planning and digging the tunnels will be very keen to avoid any of them collapsing, regardless of what buildings are above.