They have him in emails literally saying things like "If I keep entering the same clip sizes, people will become aware of what I am doing rendering my spoofing pointless" and "I need to know whether you can do what I need, because at the moment I’m getting hit on my spoofs all the time and it’s costing me a lot of money."

It's not a huge reach for the government to claim he was spoofing when he himself claims he was spoofing.

How could a small trader like that possibly move enough volume to even budge the market as a whole, never mind crash it?

I think people who aren't familiar with trading (especially futures trading) don't understand that Sarao was spoofing massive size. He was placing orders for 500+ contracts at multiple levels in the book... that's on the order of $100M notional value. He was more of a pro than people like to believe.