Ok i looked it up.
The a dive in AOWD has the same prerequisite, as the whole specialty. (Exceptions are: ice, cavern and self reliant).
The prerequisite for the deep dive specialty is adventure diver. So you need to have 3 AOWD dives (including the theorie). It is not necessary to certify the students to adventure diver, of course.
That's what my CD told me in the IDC aswell.
So deep dive is usually dive 4, because its the first dive of the second day.
You're misreading the requirements, if you're looking at PADI's Instructor Manual.
There is language that states "Student divers must meet all prerequisites and other requirements for the specialty."
However, that language ONLY applies "When using specialty diver materials for Adventure Dives not supported by PADI Advanced Open Water Diver materials...."
The Deep and Navigation dives ARE supported by PADI AOW materials. (As are several other "specialty-type" dives.)
The exception you're referring to (ice, cavern, and self-reliant) is getting at the idea that you don't have to hold an ADVANCED (not Adventure) certification to do ice, cavern, or self-reliant as one of the five AOW dives.
The only prerequisite mentioned for the deep dive as part of AOW is that the instructor should "generally assess diver knowledge, and, before going to depth in open water, evaluate the diver inwater for prerequisite skills needed to complete the Deep Dive." (And this only applies if the instructor doesn't have recent diving experience with the student.) Personally, I'd almost never do the deep diver first because I use the first OW dive to do a serious in-water assessment. Typically, students are doing PPB or Dry Suit as this dive, so I make sure they have their buoyancy dialed in either way. However, if I was training somebody I regularly dove with, or perhaps who had just completed OW with me, I wouldn't flinch at doing deep as dive 1.
Keep in mind also that the "Deep" dive in AOW is NOT the same as a deep dive in the Deep course. The former is limited to 30m/100 ft., the latter to 40m/130 ft.
To sum up: You don't have to first be an Adventure diver to do the deep dive associated with AOW. If your CD told you this, they're wrong.
I'm curious, would you issue both an Adventure Diver and an AOW card to students who followed your protocol? You'd have to electronically submit the PIC after dive 3 and before the deep dive to meet your requirement.