I think 911 did not ask about the victim because it was an officer involved shooting and a completely different 911 protocol kicked in.
The police did arrive first. This is probably just because they were closer than EMS. I don't think they sent cops just because she is a cop. I think they sent cops because AG asked for them but mostly because it was a shooting.
Here's part of the call. I don't know what 911 would have done if AG answered the question, 'no I don't need police'. I'm guessing they would have been sent anyway.
Operator: Dallas 911. This is Carla. What is your emergency?
Guyger: Um. Hi this is um off-duty officer. Umm, can I get, I need EMS, uhmm, I’m in number
Operator: Do you need police as well or just EMS?
Guyger: Yes. I need both.