There is some standards for car parking, for example at reading the stadium holds 24 thousand people and the car park at the stadium holds just 1000 cars and thats strangely considered good, I dont get it either tbh.
Wembley's more than just a football stadium, while it has had its bad press its an amazing arena and will benefit sports in this country greatly. If things were to be turned around I'm sure many people would say why spend 12 million on a ride when it could be spent on much more.
The power can and has stoped before, rides which need it to get riders down such as slammer would have emergency source of power to be able to just rotate the ride to the right angle and lower.