The warrant might well work for SMS/call data... but what about What's App, or Facebook Messenger, or the myriad of other internet based communication apps. The fact the app was communicating with Facebook/What's App at the time proves nothing. You'd need the device itself to see if a message was actively being sent, or, a very expensive and lengthy multinational investigation to get the raw data/logs from Facebook/What's App themselves.
The Police don't interpret data: they collect it. A court would ultimately provide the interpretation and decide whether or not a party is guilty and this might well include testimony from experts in the field, rather than trained officers, if required.
Their statistics need convictions? Really? Take the tin foil hat off for just a moment and consider the possibility that maybe individual officers aren't incentivised by conviction rate and that even if they were, there's also an awful lot more to their job than car crashes and idiots behind a wheel. The vast majority of jobs an officer attends in any given day involve basic conflict resolution/protection of vulnerable people and prosecution plays either no part, or forms the final piece and last resort in a complex resolution.