My understanding is that they definitely don't have to be signed off by an AI. An Advanced Diver + OWI is sufficient for everything but AT1, AP1, and AP2.
Thinking about why these boxes are there, I believe it is so that as an AD you have experienced a wide range of conditions. As such I do not think the instructor has to be your buddy, but they do need to be satisfied that you did the dive correctly.
[EDIT]This was discussed on the Bsac forum earlier this year (but this was SD getting dives signed off for DL)
http://www.bsacforum.co.uk/forums/in...osts/4220.html
I don't think we came to any firm conclusions.
Janos