Get your suspension of belief mindset ready for this one...
There's no way in hell any prosecutor in the entire country would have did a plea bargain with a confessed serial killer - at best the only bargain would have been life without the possibility of parole instead of the death penalty.
Exactly how did they know what time the transport was leaving? This information is highly guarded for obvious reasons.
So their entire plan hinged on the transport driver stopping to get coffee? What if they got coffee already? Driver didn't drink coffee?