What does authority really matter? It's not like it was a heavily guarded area, and if Arnold can take control that directly, any precautions like not being able to see the building should be overriden given this hyposthesis has a host murdering a person.
Bernard's the last person to see her alive, the last to talk to her, and would be missing an alibi. The risks of him doing it outweigh the rewards. But like I said, I think that was more just a dramatic touch. The only practical reason I can see Arnold choosing Bernard specifically is if he did it to actually put the spotlight on Bernard (if he is after all a host made in his image, it probably doesn't sit well with him).
Ford has no motive, but neither really does Arnold as far as we know yet. Not likely he would be too keen on Delos corporate getting data either.