Sounds like you are doing it wrong. Once you are the attacker you can measure and check arc to all the possible targets you have.
Technically at that point you aren't the attacker, you're the Active Ship. There's a big difference.
While the you are correct that being the attacker and the active ship are not the same thing, I believe what I wrote is accurate. You become the attacker once you enter the Declare Target step and it is in that step that you are permitted to measure and check arc. If you had to be the active ship in order to check range and arc, Corran and Dengar's abilities would not work.
But the timing of those would be likely filed under resolving abilities, not declaring attacks.
EDIT: Also, I went back and checked the flow chart, and your answer is literally the first line in the flow chart: "Measure range to enemy ships and check attacker's firing arc". Ships. With an 's'. Also also, it seems i got a bit lost in the back and forth and I'm apparently posting this to the person you were replying to, not to you. But my above answer is to you, and I also checked that and I am right: in cases like Dengar you are resolving an ability, at which point you start all the way back at "declare target" with the addition that the target of that attack must be a certain ship.
Edited by That One Guy