@brennen.exe:
You responded to my original post under the assumption that I was pointing out the presence of other pirates, I think.
Oh my bad, I assumed that you were showing the Buggy pirates to display that it made no sense they could get close to ID and not BB because I had seen that reasoning before.
I just wanted to clarify that the point of the link was to show that you need to radio in and identify yourself, as well as give a code, before they will prepare and open the gates. Blackbeard did neither of these.
Phone: This is Momonga, Marine Code G100660. I'll be in the current soon. Prepare to open the gate.
I think he did, offscreen. Ok I read all your other arguments so I understand your position better but I will say how I see it, and this way it makes sense too which I think gives it precedence over just thinking Oda messed up.
Blackbeard used his title to get on a battleship without rousing suspicion (he's an ally of the marines now), takes over the crew, beats the passcode out of the captain and uses this code to get through both the MHQ gate and ID gate. If he hadn't become a shichibukai, he would have had to attack or steal a marine ship, which means alarms would have been raised since he would be just another pirate, and he could have never used a code to get through the ID gate. So now they let him through the ID gate (unknowing at this point that BB simply commandeered the ship), and are waiting for him as he docks to curse him out that they weren't expecting him and he shouldn't have been there. It shouldn't be too out of the ordinary though, considering Garpe also probably just goes where he pleases.
If it had been Luffy that had given BB the shichibukai title, he could have done the same thing basically, just hitch a ride on a battleship for some bogus reason, no alarms are going off, and at an opportune moment take over the ship and use the code to get through the ID gate. Then free the prisoners, and sail off to where-ever Whitebeard is to try and get his power, or use another ploy to draw him out.
But now the situation is different because WB is at MHQ so he needs a way to get back there, and that is why Lafitte hypnotized the guards, not to leave MHQ but to get back there on his return trip.
Another point, considering he got defeated by Magellan, if it wasn't for Luffy's escapades his whole plan would have failed. On the one hand I feel the same, on the other hand, well he went with "fate" but I like to think that he mainly lost for plot purposes anyway, because if he had already taken down Magellan there, the whole conclusion of the ID arc would have been moot.