By the way, it's not just the Paralympics that extend the date on, it's the arrival of all the officials, contestants, medical, support and govt hangers on from dozens of countries arrives in an increasing stream, that brings the date forward too. It's not just 'three weeks'.
For Paralympics, multiply number of contestants by x, reduce spectators by y - not sure what the actual factor is, but it is a different loading compared to the main event due to all the different categories.
Even with a peak loading during the 100m final, London
should still be able to cope, as many cities far smaller have held the Olympics in the past. At it's busiest, what effect is 100k on a system that handles 3m+ people per day - surely the loading problems are the pinch-points around the event locations and key junctions.
I believe the govt is claiming hs2 will also result in an extra 100k or so visitors to London each day - but only around 2% extra load on the tube (network wide). Simples!