Is it a standard procedure to use emergency slides without imminent danger?
If an evacuation is called for the quickest method must be used, in this case slides. No exceptions no questions. Passengers can complain after they are out but they are not given a chance to argue during the evacuation, force can be used to clear an argumentative passenger so that the other passengers can get out. The 30 minute delay was before the decision to evacuate, it has nothing to do with the actual evacuation. Full scale evacuations require no more than 90 seconds for the aircraft to be certified.

What the police need is completely irrelevant to the evacuation.  

Slides can be inspected, repacked, and reused, they are test deployed at regular D maintenance inspections.(maybe C as well. D is every 6 years C is every 2 years) Emergency doors are also routinely opened for testing.

A passenger needing medical attention could very well be nothing more than a stubbed toe, light scrape, or being out of breath.