In any case, we know Officer Sabastian mentioned the officer.
No, "we" don't
know that. For one thing, it says "75(?)". For another, it sounds like "that the office" or "at the office" to me. But if Sebastian actually heard that Tippit was DOA, then why would he be asking "that the officer?" in the first place?
And might possibility (but probably not, probably Officer Owens) also mentioned the President.
To my ear, it sounds like:
A3 (or 83) on the president
No, that's not correct. Stand by, 19.
Why should this be considered only talking about the death of the President. Wouldn’t it be more reasonable to say that Officer Sabastian was only talking about the death of a police officer. Or at most the death of both a police officer and the President?
No, because there is no evidence of any radio report of a dead policeman at that time. There is evidence of a radio report of JFK's death at that time.
A slightly garbled report, that the President was DOA, should not cause one to conclude that this could not possibility be referring to the death of President. Any more than a slightly garbled report, that a police officer was DOA, should not cause one to conclude that this could not possibility be referring to the death of a police officer.
But there is no "slightly garbled report that a police officer was DOA". That's just the spin you're trying to put on it.