Also, hundreds of mobile hotspots will impact the performance of all nearby wireless networks.

I wonder if Google actually has WiFi engineers. It’s not something you need to set up everyday. Still they have the money to hire a company that specialises in WiFi.

I don’t think the roof would be good at reflecting signals back at the device, it scatters them all throughout the building, rasing the noise floor. In a way, phone hotspots can cause less interference then a proper access point because they use a lower transmit power, and allow the other devices to reduce power.